58 replies to this topic

[Menace]

Hey guys, I don't post too often on this forum (but do read it ;) ) but I need some help with my baby!

I have a 71' Firebird with a bick-block 400, that needs some help.

Here's the situation: I recently purchased a high-performance ignition system for the car, this includes the best MSD pro-billet distributor made for my car, a MSD digi-box, the best coil and spark pluck wires out there on the market (both MSD as well). I also recently purchased a Holley Avenger carb, that sits on top of a Edelbrock Performer intake.

The problem: Ever since I installed the system on my car, the engine hesitates/try's to stall at lower RPM's while under load? It is not noticable at higher RPM's and speeds, but very annoying especially in heavy traffic! I have tried to trouble-shoot for obvious things such as fuel delivery system (I also bought a new Holley Avenger carb), vacuum leaks, carburator tune-up & timing, looking for possible surge sources on the ignition system, and I cant find anything?

Could it be that when I dropped in the distributor it was not facing properly towards #1 cylinder? Could it be my timing-belt is slipping? I'm fairly clueless when it comes to cars besides the stuff that I have learnt on my own time messing with my car.

Any help at all would be greatly appreciated.

[Greg Locock]

Well I'm glad you bought a billet distributor, I must admit that's the first thing I think of when working on an engine.

So, have you checked the timing? With a timing light? If that is right, and the lead from no.1 on the cap goes to cylinder number 1, then the dizzy must be pointing the right way.

Is the intake single plane? Was your old one dual plane?

If your timing belt was 'slipping' you'd have bigger problems than just hesitation at low revs.

What cfm is the carb? what cfm was the old carb? Carbs are matched to performance, it is very hard to get a carb with a high flow rate that also works at low flow rates.

Did you change the camshafts?

[Chubby_Deuce]

This sounds like an induction issue more than a timing issue to me. Did the engine have any bad backfires through the carb when you were setting the timing after installing the new components?

Also, I'm sure your engine uses a timing chain which would have destroyed your pistons and valves by now had it jumped any teeth.

[Menace]

Greg,

I have checked the timing with a timing light, and that is one of the reasons Im a little confused.

The engine seems to run best at 18-22 degrees BTDC even though the factory spec should be 12 degrees.

I dunno about the intake, all I know it's an aluminum Edelbrock performer intake.

The carb is 670cfm brand new Holley Avenger, which really shouldn't be too big for a 400, right? I have never touched the camsaft as of yet, gladly!!

[Menace]

Quote

Originally posted by Chubby_Deuce
This sounds like an induction issue more than a timing issue to me. Did the engine have any bad backfires through the carb when you were setting the timing after installing the new components?

Also, I'm sure your engine uses a timing chain which would have destroyed your pistons and valves by now had it jumped any teeth.

Well Im glad to hear it's not the timing chain, I din't feel like having to replace it!

As far as I can remember the engine fired immediately when I dropped the new distributor in there, and there have been no backfiring with the new carb.

Could my magnetic pick-up lines be interferring with the coil??

[McGuire]

If you give me a more complete description of the problem, I am sure we can diagnose it here.

First, when the engine stalls at lower rpm do you mean when accelerating at partial throttle, or at steady speed or foot off the throttle? Does the engine miss or lose power first, or just die? If you get tricky with the gas pedal, can you catch it and save it?

Does the engine backfire? If so, through the intake or out the exhaust?

How does the engine run at full throttle and/or high rpm? Does it pull cleanly when you stuff your foot all the way in it?

Be descriptive!

[McGuire]

Quote

Originally posted by Menace
Greg,

I have checked the timing with a timing light, and that is one of the reasons Im a little confused.

The engine seems to run best at 18-22 degrees BTDC even though the factory spec should be 12 degrees.

I dunno about the intake, all I know it's an aluminum Edelbrock performer intake.

The carb is 670cfm brand new Holley Avenger, which really shouldn't be too big for a 400, right? I have never touched the camsaft as of yet, gladly!!

What are the distributor's advance curve specs? Does this distributor have mechanical advance, vacuum advance, both, or neither?

[Menace]

Quote

Originally posted by McGuire
If you give me a more complete description of the problem, I am sure we can diagnose it here.

First, when the engine stalls at lower rpm do you mean when accelerating at partial throttle, or at steady speed or foot off the throttle?

Partial throttle.

Quote

Does the engine miss or lose power first, or just die? If you get tricky with the gas pedal, can you catch it and save it?

Te engine doesn't miss, but it looses power for a second, and so far has never actually died. If slam the pedal past 1/4 of the throttle you wouldn't really even notice it, but on a smooth slower acceleration you can really feel it.

Quote

Does the engine backfire? If so, through the intake or out the exhaust?

The engine doesn't backfire at all...

Quote

How does the engine run at full throttle and/or high rpm? Does it pull cleanly when you stuff your foot all the way in it?

The engine runs strong and smooth at a full throttle and high rpm, and it pulls cleanly if you stuff it.

Quote

What are the distributor's advance curve specs? Does this distributor have mechanical advance, vacuum advance, both, or neither?

The distributor has mechanical advance, and a magnetic trigger. Here's a link for specs: http://www.msdignition.com/dist_35.htm

...and heres a link for a pdf fir the same distributor but for a Chevy V8: http://www.msdigniti..._8547_dists.pdf

Hope that helps, I really hope you can help me diagnose this! Thanks for all the replys!

[Greg Locock]

Sounds a bit like the jetting or the accelerator pump. The intake is a dual plane so that's OK. The carb is a bit big (300-400 hp, unlikely with a dual plane manifold, the 570 cfm is recommended), but not ridiculous.

I've never seen a Holley, I don't know how the part throttle enrichment system works.

http://shop.store.ya...e/acpumsys.html

http://shop.store.ya.../powvalsys.html

seems to be talking about the right sort of thing. Maybe your secondaries are opening incorrectly.

Oh, the way we'd describe that is 'tip-in hesitation at low speed'

[Menace]

Greg, thanks a lot! That could be it, I will keep you guys posted on my progress!

[Chubby_Deuce]

It would be very sad if the accelerator pumps were dead on a new carb.

[Menace]

Yes it would! It would certainly make me wonder why I didn't just buy another Edelbrock!

However it could be I just need a larger pump nozzle as indicated in the first link...

Quote

During acceleration tests, if you notice that the car first hesitates and then picks up, it's a sure bet that the pump nozzle size should be increased

I will play with the pump tomorrow if I have time...
hopefully it's something relatively simple like that!

[Chubby_Deuce]

That would be what I would do. Sounds like it's just loading up the cylinders when you leave the line. It's fairly easy to mess with on an Edelbrock so I'll just assume that it's about 20 times harder on the Holleys.

BTW Menace, where are you from? I had assumed that you were Finnish but I don't know of any with 71' Firebirds. American? Canadian?

[Greg Locock]

" It would be very sad if the accelerator pumps were dead on a new carb."

Oh, maybe it wasn't obvious.

All of those things are tunable.

If the carb has not been set up for a particular engine, and in this case there /is/ a mismatch between the carb and the usage, then the acelerator pump, the secondaries, and the power valve (hoho) can and should be retuned. Basically you've got a high end street carb fitted to a mild street engine.

[McGuire]

I think the first thing we need to do is sort out the timing issue, before we zero in on the carb. If the static/idle timing really is 18-22 BTDC, you are going to have a hot-cranking problem for one thing. Once we get the distributor sorted we will have to retune the carb anyway, so let's start here first.

Base timing should be around 12 BTDC, with around 36 or so total (rough ballpark). I see this distributor has an adjustable mechanical advance. Let's make sure it's at zero advance when we are setting the base timing, then (assuming we don't have a distributor machine) use the timing light to adjust the mechanical advance in the distributor so it's in by 3000 rpm or so (again, ballpark). 12 degrees at idle, 32-36 by 3000 rpm is a decent rough baseline.

Since this is a 1971 engine and a lot can happen in 30 years, let's also make sure that this engine has the correct crankshaft damper and timing indicator, and also that the damper ring has not slipped on the hub. With the #1 piston at TDC (first cyl on the RIGHT bank on a Pontiac) and intake and exhaust lobes on their base circles (both valves closed and rocker arms slack) the TDC mark on the damper should be exactly at TDC on the timing cover or bracket. If not something's out of whack, and you would be amazed how often it happens as stuff wears out or parts are replaced over the years.

While we are at it, it so happens that 1971, lucky you, was the last year for Pontiac's infamous nylon camshaft sprocket (in 1972 they went to steel). The nylon sprocket is the source of all the Pontiac timing chain problems of song and story...it would be rather unlikely that yours hasn't already been replaced , but you never know. With all the spark plugs out and the distributor cap removed, put a breaker bar on the crank bolt (1 1/8" if I remember properly) and rotate the engine forward and back, while watching the distributor rotor and feeling for slack or snatch in the chain on your wrench. If in doubt, take it out. It's an easy job to pull the cover and replace the chain and sprockets if required.

[McGuire]

Quote

Originally posted by Menace
Yes it would! It would certainly make me wonder why I didn't just buy another Edelbrock!

However it could be I just need a larger pump nozzle as indicated in the first link...



I will play with the pump tomorrow if I have time...
hopefully it's something relatively simple like that!

The Holley 670 Street Avenger is a nice match for your engine. The Holley 4150 series is nearly infinitely adjustable anyway, so no worries.

On the accelerator pump, there should be .015" clearance between the pump lever and the actuator screw at idle. Also, a number of pump housings and plastic actuator cams are available to tailor the pump shot. However the accelerator pump will only affect the first few instants or so of throttle opening, so it's probably not your problem. Most of the part-throttle tuning is accomplished by changing the power valve in the primary metering block, and by altering the opening rate of the secondaries. Is this car a stick or an automatic?

[Menace]

Quote

Originally posted by Chubby_Deuce
That would be what I would do. Sounds like it's just loading up the cylinders when you leave the line. It's fairly easy to mess with on an Edelbrock so I'll just assume that it's about 20 times harder on the Holleys.

BTW Menace, where are you from? I had assumed that you were Finnish but I don't know of any with 71' Firebirds. American? Canadian?

I AM Finnish! I have lived in U.S. happily with my American girl-friend for some 4+ years now though!
;)

[Menace]

Quote

Originally posted by McGuire


The Holley 670 Street Avenger is a nice match for your engine. The Holley 4150 series is nearly infinitely adjustable anyway, so no worries.

On the accelerator pump, there should be .015" clearance between the pump lever and the actuator screw at idle. Also, a number of pump housings and plastic actuator cams are available to tailor the pump shot. However the accelerator pump will only affect the first few instants or so of throttle opening, so it's probably not your problem. Most of the part-throttle tuning is accomplished by changing the power valve in the primary metering block, and by altering the opening rate of the secondaries. Is this car a stick or an automatic?

Automatic! Holy moley, looks like this will be alot of work! I could have done something to the timing chain/cover when I had to pull my harmonic balancer, maybe this is the original source of my problems.

Re: the timing; I cranked it up to make the sluggish spot not so noticable while driving at lower RPM/ citi traffic. I have had no hot cranking problems so far, and the engine turns fine even when hot.

Thank's again for all the advice, I feel confident that with enough tickering and time I can hopefully find the source of my problems. Lucky me, if I have to mess with the timing chain etc.

Greg, I went to the local Baxter's and they assured me the carb should have been tuned for my engine already. 71' Firebird with a 4bl 400 V8 should push stock about 300hp. Considering all the ignition modifications and all the tinkering, I would be very suprised had I not broken the 300 mark already!

[Menace]

Quote

Originally posted by McGuire


The Holley 670 Street Avenger is a nice match for your engine. The Holley 4150 series is nearly infinitely adjustable anyway, so no worries.

On the accelerator pump, there should be .015" clearance between the pump lever and the actuator screw at idle. Also, a number of pump housings and plastic actuator cams are available to tailor the pump shot. However the accelerator pump will only affect the first few instants or so of throttle opening, so it's probably not your problem. Most of the part-throttle tuning is accomplished by changing the power valve in the primary metering block, and by altering the opening rate of the secondaries. Is this car a stick or an automatic?

In fact I do think it is (atleast I hope so), cause like I said besides for the first 1-2 seconds on the throttle the engine seems to run perfect. There is no hesitation on higher RPM's past the 1,400 or so.

[McGuire]

Quote

Originally posted by Menace



Re: the timing; I cranked it up to make the sluggish spot not so noticable while driving at lower RPM/ citi traffic. I have had no hot cranking problems so far, and the engine turns fine even when hot.

That it runs better with 18-22 degrees base timing tells me the mechanical advance is miscalibrated or inoperative, or the timing marks are inaccurate.

[Menace]

Quote

Originally posted by McGuire


That it runs better with 18-22 degrees base timing tells me the mechanical advance is miscalibrated or inoperative, or the timing marks are inaccurate.

Ok timing back to 12 degrees, engine idles better but the sluggishness hits now at 1,700rpm

I have tried two new squirters on the accelerator pump and it makes little difference...

[McGuire]

Quote

Originally posted by Menace


Ok timing back to 12 degrees, engine idles better but the sluggishness hits now at 1,700rpm

I have tried two new squirters on the accelerator pump and it makes little difference...

If you have replaced them twice, the nozzle(s) are probably not the problem. With the engine shut off/choke plate open, look down the primary throats while rapidly opening the throttle. If you can see two jets of fuel squirt from the nozzles into the venturis the pump is fine.




If there is very little or no spray, the trouble is probably a split accel pump diaghram, a check ball is stuck/missing or the passage is plugged. (On a new carb this is very rare.) Also: on some Holley 4150's, if the bowl or metering block gasket is reversed the pump passage will be blocked off.






If the accel pump appears to be working okay, proceed to the distributor. With a timing light, you should be able to see the timing mark climb away from the base setting and disappear out of sight as you rev the engine up to 3000 rpm. If it does not, there is your problem right there -- no spark advance.

For about $60 to $75 you can remove the distributor and have it set up and calibrated on a distributor machine by a qualified ignition specialist or speed shop. Tell them you want 28 degrees of mechanical advance in by 3000 rpm, 16-18 degrees by 2000 rpm. (28 instead of 24 will give you more adjustability of the base timing.) These are probably the specs the shop will suggest anyway.

When you get the distributor back, set the base timing initially at 8 BTDC (36 degrees total), with the option of bumping it up to as much as to 12 degrees (40 degrees total) if you find the fuel etc. can handle it. (Beware of any audible spark knock, especially if you run prolonged WOT). If it doesn't run any better at 12 BTDC than at 8 BTDC, run it at 8.

If you feel confident enough, it is not that difficult to calibrate the mechanical advance yourself with a timing light...just a little time-consuming. Did the new distributor come with an instruction sheet or booklet?

[McGuire]

I see from the website this distributor is equipped with a feature for locking out the mechanical advance. Make sure it isn't engaged. (it doesn't say how but it shouldn't be too hard to figure it out -- probably a pin(s) holding the weights.) Also, your distributor should have come with a packet of small springs and bushings. Install the red bushing on the stop pin and the two light silver springs on the advance weights according to the instructions and you should be very close, according to what I can see from the advance curves pictured on the site.

http://www.msdigniti...ist_mechadv.htm

[McGuire]

Carburetors, distributors, mechanical advance springs and weights...I'm sure Mr. Locock finds all this very quaint.

Reminds me of an amusing conversation I had with John Fernandez, who runs the Dodge NASCAR program. Going into their second year they realized they had a carburetion problem. (NASCAR runs the same basic carb we are dealing with here.) So they started looking around the corporation for some real carburetor expertise...turns out they didn't have any. Chrysler built its last carbureted automobile sometime around 1988, and they had all retired! So they called up a few retirees....

[Greg Locock]

In my first job I did work on a carburetted distributored engine... in fact I can remember the glee with which we fired up our first fuel injected V8 on the dyno. We still had carbs on the V8 LandRovers for several years after that - with an orifice plate in them to restrict the power output. It didn't take a genius in the garage to realise that taking the orifice plate out might be a good idea.

And of course at home it's only 10 years since I made the (long awaited) move to electronic ignition. Fortunately, since my daily driver is a Toyota, I haven't had to touch the carb, so far. I think it'll comfortably outlast the bodywork, which is rapidly turning into lacework. It seems odd to scrap a car from sheer boredom, but that is its likely fate!

[275 GTB-4]

Fascinating stuff guys!!

Can I just say.....Mercury Cougar, Pontiac Firebird 400, Z28 Camaro, Shelby 500 Fastback Mustang, GTO....you lucky lucky people!!!

[Big Block 8]

Nice set of wheels you have there Menace!

My 2 cents in addition to McGuire's input. Check that the vacuum hose from the distributor vacuum advance to carburetor is connected into the correct port in the carb. There should be more than 1 port available (at least 2 with Rochesters and Carters, and Edelbrocks, which are rebadged R's and C's). Different ports give different vacuum depending on rpm. Check the manuals for correct details and make sure to plug the ports not needed.

[Menace]

Quote

Originally posted by McGuire
I see from the website this distributor is equipped with a feature for locking out the mechanical advance. Make sure it isn't engaged. (it doesn't say how but it shouldn't be too hard to figure it out -- probably a pin(s) holding the weights.) Also, your distributor should have come with a packet of small springs and bushings. Install the red bushing on the stop pin and the two light silver springs on the advance weights according to the instructions and you should be very close, according to what I can see from the advance curves pictured on the site.

http://www.msdigniti...ist_mechadv.htm

Thaks McGuire. Im going to test the springs right now, will post back later on results!

[Menace]

Quote

Originally posted by Big Block 8
Nice set of wheels you have there Menace!

My 2 cents in addition to McGuire's input. Check that the vacuum hose from the distributor vacuum advance to carburetor connected into the correct port in the carb. There should be more than 1 port available (at least 2 with Rochesters and Carters, and Edelbrocks, which are rebadged R's and C's). Different ports give different vacuum depending on rpm. Check the manuals for correct details and make sure to plug the ports not needed.

Thanks Big Block 8, but I have a mechanical advance no vacuum line coming off the distributor.;)

[Big Block 8]

Quote

Originally posted by Menace


Thanks Big Block 8, but I have a mechanical advance no vacuum line coming off the distributor.;)

Well, at least you solved the distributor vacuum problem quickly.;)

[Big Block 8]

Quote

Originally posted by McGuire
Carburetors, distributors, mechanical advance springs and weights...I'm sure Mr. Locock finds all this very quaint.

Reminds me of an amusing conversation I had with John Fernandez, who runs the Dodge NASCAR program. Going into their second year they realized they had a carburetion problem. (NASCAR runs the same basic carb we are dealing with here.) So they started looking around the corporation for some real carburetor expertise...turns out they didn't have any. Chrysler built its last carbureted automobile sometime around 1988, and they had all retired! So they called up a few retirees....

A few years ago, I tried to get some help with a carburetor problem, so I picked up a phone book and found a company called "The Carburetor Experts". So I called them - and got a response "we stopped selling carburetors and parts in 1994, we don't know nuthin' about them no more". The company is still there today, with the name unchanged.

[McGuire]

I can elaborate on the vacuum plumbing picture. Essentially, there are two basic "kinds" of vacuum ports on a typical carburetor: "manifold" and "ported" vacuum (the latter also known as "venturi vacuum").

Manifold vacuum is ported from beneath the throttle plate. There is high vacuum (low relative pressure) at idle which decreases to low vacuum (near-atmospheric pressure) as the throttle is opened. It's just like tapping directly into the manifold plenum itself.

Ported or venturi vacuum works in reverse: here the passage is ported to the carb throat above the throttle plate, so there is little or no vacuum at idle, and inceasing vacuum with increased throttle opening.

So if you connect a vacuum-operated device to the wrong type of port, it will work in reverse from intended: For example, the vacuum-advance distributors on many Fords, or the EGR valves on most makes, are designed to operate on ported vacuum signal. If you hook these devices up to manifold vacuum, they will work exactly backward. For example the EGR valve will be open at idle and closed at open throttle. (A bad thing: the engine probably won't idle at all.)

Some distributor vacuum advances are designed to work on manifold vacuum; others (especially Fords) on ported vacuum. If you connect the advance to the wrong vacuum source, the vacuum servo will rotate the distributor's advance plate in the wrong direction, advancing the timing when it is supposed to ******, and vice versa.

There are some other "kinds" of vacuum found in the vacuum-hose harness of a late-model car -- many vacuum-operated devices are also thermostatically controlled, and there are even positive-pressure circuits on supercharged and turbocharged cars...but we don't have to get into all that here.

Vivian's MSD distributor is not equipped with vacuum advance. In my opinion this is slightly less than ideal -- on street engines, all but the most extreme-performance applications really should use both mechanical and vacuum advance (unless they run electronic spark control of course). However, we can work around it pretty well with the proper mechanical advance curve. There will be little effect on performance or driveability, but fuel economy will suffer somewhat.

[Chickenman]

Quote

Originally posted by McGuire


The Holley 670 Street Avenger is a nice match for your engine. The Holley 4150 series is nearly infinitely adjustable anyway, so no worries.

On the accelerator pump, there should be .015" clearance between the pump lever and the actuator screw at idle. Also, a number of pump housings and plastic actuator cams are available to tailor the pump shot. However the accelerator pump will only affect the first few instants or so of throttle opening, so it's probably not your problem. Most of the part-throttle tuning is accomplished by changing the power valve in the primary metering block, and by altering the opening rate of the secondaries. Is this car a stick or an automatic?

Gack....sorry wrong advice on pump lever setup. Proper procedure is to adjust accelerator pump lever spring untill there is ZERO clearance between the accelerator pump arm and pump cam at idle. Then add 12 turn of preload to the spring.

The .015" figure so often quoted is a checking figure at WOT to make sure that you do not over-travel the pump diaphram. Seldom happens, unless you put a 50CC cam on a 30CC pump. After properly adjusting pump lever to Zero clearance with cam at idle, open to WOT ( Engine off of course) then measure the clearance between the bottom of the pump spring pad and the diaphram lever, while manually pushing down on the diaphram lever till the diaphram bottoms. You should have a minimum of .015" clearance between the spring pad and the diapharm lever. It is really only a safety check to detect over-travel and the .015" figure is NOT used in any way for actually adjusting the pump clearance.

I've been tuning these carbs for years ( Along with Webers, Q-jets and Su's ) I generally work on SBC's and Poncho's. I'm pretty sure I can give you a baseline setup...but PM me if you want to go into detail so we don't use too much bandwidth. There are Forums for this. I'm Chickenman35 at www.thirdgen.org . You'll find me as one of the carb Guru's at the " Carb Board".

Here are a couple of links that should help you.:

http://www.thirdgen....threadid=208836

http://www.thirdgen....threadid=202171

Baseline setup for Poncho 400:

Actually a 670 Avenger is a bit small for a 400 CI motor, particularily on a Dual Plane manifold such as the Performer. You could have easily used a 750 CFM carb. In addition, with a Vacuum Secondary carb such as the Street Avenger, it is virtually impossible to over carb. I used a ( highly modified ) 780 CFM Holley 3310 VS carb on my 305 ( now 430 HP 355ci )....with incredible response and driveabilty. VS carbs are great on the street.

IMPORTANT!!!! MAKE ONLY ONE CHANGE AT A TIME. And record the results in a log book so you can accurately compare data.

1: Accelerator cam choice is critical. More so than pump nozzle size... although the two work together as a unit to deliver the fuel. Holley puts a baseline cam in their models...that is usaully too weak. Particulary on heavy cars with Auto's. Articles above will tell you why pump cam selection is so critical.

Buy a Holley pump cam kit #20-12. Install the Blue cam in the #2 hole ( Note matching number on carb linkage. #2 hole on cam must go to #2 hole on linkage ) and adjust clearance useing the method I stated. You didn't state what gear ratio you have so I'm going to go a tad big from what I'd normally recommend. Start off with a #35 shooter. Get the tube type they are more accurate. Also get a #31 so that you go down in size if necessary. Part# 121-31 and 121-35.

2: Check the float level. Proper level should be just barely visable on the lower edge of the clear site plug.

3: Adjust idle mix. With car fully warmed up, connect a vacuum gauge ( undamped ) to manifold vacuum. Adjust mixture to highest steady reading. Lean out the mixture, 14 turn at a time ,until vacuum reading just starts to waver or drop. This is " Best idle". 14 turn leaner would be " Lean Best idle". 14 to 12 turn richer will be " Rich best idle". You want to set up your car for " Rich Best idle" for best driveabilty and mileage. For emissions testing, you may want to adjust to " Lean Best idle".

4: Carb jetting. Leave this to last. The Avenger series is jetted pretty lean for Emissions. Your early model non-emission engine is going to find this a bit too lean. The small (670 cfm on a 400ci Big Block Poncho ) carb makes matters worse. Likely you'll have to jet the Primary side up two jet sizes and maybe the Secondary side as well. But we'll leave this to last. You can PM or e-mail me once we really get into things.

5: Distributor choice is a big problem. For a Drag Race or Race track car only, your distributor choice is fine. But for a street driven car, you ABSOLUTELY must have a vacuum advance to tailor the timing curve correctly for driveabilty and milage. Lack of vacuum advance can also cause plug fouling. Vacuum advance function is often misunderstood....particularily by " Drag Racer" types ( They're all Neanderthal knuckle draggers from us " Road Racers " perspective LOL ). Numerous books on this. Particularily good are the books by " David Vizard", "How to Build Horsepower Volumes 1 and 2" . My advice is to buy thes two books and read them cover to cover. Get some books on Holleys as well. One of the best is Holley Carbs by Dave Emanuel, ISBN 0-931472-08-3.
If out of print, pick up a used copy from Amazon.com.

Back to your Distributor. If you can , return it in exchange for a vacuum advance model. Since it's been used that may not be an option, so we'll have to go with a work around.

A vacuum advance increases the timing when the engine is at light load, low throttle openings such as cruise or in town. More advance is required because the throttle butterflies are barely open, this decreases the charge density, which require a longer time to burn. Note that there is a difference between charge density and mixture strength. They are not the same.

At idle a SBC, BBC or Poncho motor often needs 25 to 30 degrees to idle " Optimally". 12 to 16 degrees static advance, plus 15 degrees vaccuum advance. At cruise , American V8's can tolerate ( and may require ) as much as 40 to 50 degrees, because the charge density is so low ( volumetric efficiency often down to 20% to 25% ) and the mixture needs to be lit a lot earlier. Put your boot in it....Charge density goes up, Dynamic compression goes up and suddenly you need less total timing. Vacuum advance takes care of this because it is " Load " sensitive. As soon as you give it some moderate to heavy throttle, manifold vacuum drops and the additional advance provided by the vacuum advance drops off and you are now back to the mechanical curve only.

So for now. Without a vacuum advance you are going to need a ton of static advance at idle. That is why the car runs better with 18 to 22 degrees of advance. It really wants 25 to 30...but you're missing the 10 to 15 degrees normally supplied by the vacuum advance. That's also why things seem fine at WOT or when you really punch it, as oppossed to light throttle opening at cruise rpm. At WOT or heavy loads the vacuum advance drops out of the equation.

Set the static timing as far advanced as possible and stiil start the car hot. Somewhere around 18 degrees will probably be the max.

Idle the engine down to 750 rpm. Slowly increase speed. You do not want the mechanical advance to come in before 1,000 rpm. Any sooner and idle will become unstable. Adjust with springs as per MSD instructions to start mechanical advance at 1,000 rpm...no sooner!!. You want full mechanical advance all in by 3,200 to 3,500rpm. Shoot for 36 to 38 degrees total timing. EG: 18 degrees static timing plus 18 degrees mechanical advance =36 degrees total, or 18+20=38. Adjust stops in MSD distributor to limit total mechanical.

That should get you going as a baseline. Remember. MAKE ONLY ONE CHANGE AT A TIME. And record the changes. Feel free to PM me or e-mail me at rboyk@telus.net or visit us a www.thirdgen.org. As this is an F1 forum, I do not want to use any more bandwidth here as a courtesy.

Hope this helps. Sorry for the length....

Edit: Spelling

[Menace]

Lots of good information Chickenman. I don't know why I dont ask questions on this board more often!


Update: Installed the two light silver springs on the distributor and it made a HUGE difference on the driveability. Thanks McGuire! However, I did not install the RED stop bushing, because it would have required me to pull the distributor (it sits on the back of the engine) and I didn't bother with it with so little time.

Anyhow, iyour advice on the springs has made a big difference, Im going for a longer ride to test it better right now.

[Menace]

BTW, the engine timing is set at 12 degrees right now, I tried 8 like yousaid McGuire, but it runs better at 12.

Chickenman, so much info! Thanks for everyone who has contributed in this thread. It's great to know there is so many smart people on this board! I will tackle issues you raised during this week, and will get back wigth results!

[desmo]

Please don't worry about bandwidth issues guys, I find this fascinating and I suspect others will as well. Carb issues bring up all sorts of interesting fundamental issues common to all spark ignition engines and this is an interesting illustration of good troubleshooting methodology as well.

Thanks.

[Big Block 8]

Thanks for the vacuum elaboration McGuire. Personally I learned it the hard way many years ago, coincidentally with a Poncho 400, as I got info from the shop, which turned out to be wrong and hence connected the hoses wrong at first, before making some research.

Menace, just a general comment regarding the title of this thread - Pontiac doesn't actually have the same kind of small block / big block separation than Chevy, Ford and Chrysler for example. Pontiac V8s of the 326-350-389-400-428-455 ci family share the same block dimensions (and practically all Pontiac V8s you stumble into are from the 326-455 family) and lot of their parts are interchangeable. Later exception is the 301 "economy" motor, which was introduced in 1979 and that was physically smaller than the 326-455. However, the 326-455 line was discontinued in 1978, so Pontiac didn't have two engine production lines simultaneously, hence no official need for big/small block separation. Those 301s weren't much to write home about in terms of power and their weak structure and small valves won't respond well into modifications either - so they're nowadays practically only found from some late 2nd gen Firebirds, whose owners want to keep their cars original.

Pontiac 326-455 engines are pretty good engines for power. They're more robust and heavier than their small-block rivals of other makes and divisions and especially the 350-400-455 versions have large valves, so labelling them as "big-block" isn't a bad metaphor. Some enthusiasts will get pissed if you start talking "blocks" in the same breath as "Pontiac", though, so beware.

[McGuire]

Quote

Originally posted by Chickenman


Gack....sorry wrong advice on pump lever setup. Proper procedure is to...

One step at a time, Chickenman. New Holleys out of the box are notorious for the pump well siphoning, eh.

[McGuire]

Quote

Originally posted by Chickenman
Actually a 670 Avenger is a bit small for a 400 CI motor, particularily on a Dual Plane manifold such as the Performer. You could have easily used a 750 CFM carb. In addition, with a Vacuum Secondary carb such as the Street Avenger, it is virtually impossible to over carb. I used a ( highly modified ) 780 CFM Holley 3310 VS carb on my 305 ( now 430 HP 355ci )....with incredible response and driveabilty. VS carbs are great on the street.

I'm sure the 670 CFM carb he already paid good money for will work fine. How big a carb can be made to work is not an issue for me. I don't worry about the size of my equipment in quite that way.

400 CID @ 6000 rpm @ 100% Ve = 695 CFM. Looks good to me. Meanwhile we have a low-compression engine, stock heads and OE camshaft, automatic with original torque converter. I don't see anything crying out for airflow volume. A bigger carb quote-unquote may provide more power above that rpm, at the expense of torque, response and tuning sensitivity everywhere else. Those secondaries have to open sooner or later...

I can appreciate this is not the carb/distributor combination you would select. Might not be my first choice either, but this is not about us. This is about getting this stuff to work right. Meanwhile I have no doubt it can. One of the amusing things about speed equipment advice: the way people talk, you would think everyone is on commission.

[McGuire]

Quote

Originally posted by Big Block 8
Thanks for the vacuum elaboration McGuire. Personally I learned it the hard way many years ago, coincidentally with a Poncho 400, as I got info from the shop, which turned out to be wrong and hence connected the hoses wrong at first, before making some research.

Menace, just a general comment regarding the title of this thread - Pontiac doesn't actually have the same kind of small block / big block separation than Chevy, Ford and Chrysler for example.

Vacuum hose routing is "simply" a matter of understanding what all those devices are intended to do: idle or PT, cold engine or warm, etc. It's not hard at all if you do it all the time...or you can always go to the local library and photocopy the vacuum schematic from the service manual. I imagine there may even be a reference resource somewhere on the web by now, though I haven't checked.

Quite true about the Pontiac BB vs. SB thing, as Pontiac V8s are all essentially descended from the same 287 CID engine introduced in 1955. (Except for the 215 BOP V8, Canadian variants, and some late 70's production shifting.) "Big block" and "small block" aren't real technical terms anyway...more like hot rod jargon like "3/4 race cam," "close-ratio" and suchlike. If everyone knows what everyone else is talking about, I suppose the terms have some value.

[Big Block 8]

Quote

Originally posted by McGuire

Vacuum hose routing is "simply" a matter of understanding what all those devices are intended to do: idle or PT, cold engine or warm, etc. It's not hard at all if you do it all the time...or you can always go to the local library and photocopy the vacuum schematic from the service manual. I imagine there may even be a reference resource somewhere on the web by now, though I haven't checked.

True indeed and although I then already knew the basics, it proved to be the culprit - I was too cocky sob and thus I left the ports unmarked when dismantling the carb (the OEM carb was too lean to work with the modifications I made, so I bought a new aftermarket one of different make, as it was cheaper) - only to find out later that I had forgotten, if the OEM distributor had used manifold or port vacuum. I called the shop and got a firm answer - which of course later proved to be wrong. Anyway, I was a bit puzzled for a while and thought there was something wrong with the carb / ignition and spent a lot of time tinkering and cursing, before I started suspecting the "correct" vacuum routings.

[Big Block 8]

Quote

Originally posted by McGuire

400 CID @ 6000 rpm @ 100% Ve = 695 CFM. Looks good to me.

Otherwise I agree, but I hope anyone won't take that as a hint that 6000 rpm is healthy for a stock Pontiac V8.;)

Excluding perhaps the Ram Air IVs and 73-74 SDs, I personally wouldn't recommend ever going past 5500 rpm with the original rod bolts, as they tend to stretch. I'd go for balancing, ARP bolts and good set of valve springs first.

[Chickenman]

Quote

Originally posted by McGuire


One step at a time, Chickenman. New Holleys out of the box are notorious for the pump well siphoning, eh.

Pump shot must be sharp and firm with any movement of throttle. This precludes any freeplay between pump cam and pump lever at idle.

Edit: Note: Only exception is on some Drag Race cars with high stall convertors, linelocks andor TransBrakes. Launches at 4,000+ rpm are common with a very large degree of throttle applied. In this situation, most of the pump shot is used up. One cure is to loosen the clearance so that there is some freeplay and to use a high lift cam. This is the only situation where you might possibly want freeplay between the Cam and Pump lever at idle. A better solution is to install a special profile cam designed with a shallow entry ramp ( Pink cam and Brown cam ) along with a 50CC pump assembly. On Road Race, most Circle Track and Street cars you want ZERO clearance, at idle, between the pump lever and pump cam. ( Source: Barry Grant Tech articles).

Checking procedure from Holley Tech, 015" extra clearance between diaphram lever and spring pad at WOT to prevent over travel to diaphram:

http://www.holley.co...nfo/TI-221.html.

Pump well siphoning usually not an issue except at substained high RPM's and with high boost venturies. High speed pullover at typical street RPM's ( below 6,000 rpm ) is usually a result of the pump weight not being installed.

Holley does makes anti-pullover discharge nozzles, but they are normally only required ( and designed for ) 4165 and 4175 Speadbore Emissions carbs that have to run very lean and have high boost venturies. In addition the 41654175 designs have the pump discharge weight located low down, in the base of the metering block. This requires the use of the anti-pullover type discharge nozzle as a considerable amount of fuel remains in the passage between the discharge check ball and the actual nozzle tip. This fuel would be sucked out of the passage if regular discharge nozzles were used on a 41654175 carb and cause a lean stumble when throttle was backed off, then re-applied.

415041604500 series do not have this problem, as the pump discharge check weight is positioned immediately below the discharge nozzle, so no fuel can be drawn from the passage ( except as designed as ane extra High Speed enrichment ).

A certain amount of High Speed pullover from the pump nozzles is designed into the fuel curve of the 4150, 4160 and 4500 series as additional high speed fuel enrichment ( As per articles by David Vizard, Dave Emanuel, HP Books and SA Books ). Webers and Mikuni Solexes also use this method of extra high speed enrichment from the pump bleeds. Circle Track Restrictor plate race motors, where venturi air flow approaches super-sonic, have issues with pump pullover...but never with your typical StreetStrip motor. At least to any significant amount.

To the best of my knowledge that is......

BTW...I have a problem composing short replies as you might have guessed .

[Chickenman]

Also, I'm recovering from a foot operation...So I'm stuck at home. Bored to tears..Nothing to do but Lurk in Forums, Watch Survivor ( Is Amie a Lezzie ? ) , lye on the couch and eat Bon Bons....

[Fat Boy]

Quote

Originally posted by Menace
Yes it would! It would certainly make me wonder why I didn't just buy another Edelbrock!

From experience many moons ago, the reason I wouldn't buy an Edelbrock is that as soon as you get into a corner one side of the carb goes rich, the other goes lean, and the engine dies. That carb is fine for going in a straight line, but seems to give up the ghost at about 0.5 lateral G. I liked the tuning features, none of them would make it live in a corner.

It sounds like you have a lot of help sorting this thing out, but one thing that I would like to mention is that the 'Street Avenger' has some quick adjustment that you can make to the powervalve which is supposed to either get better mileage or make more power. Is it possible you are on the 'mileage' adjustment and running into an issue there?

On the accelerator pump side, I had a 'tip-in' problem that was too much of a pump shot. If was a 600cfm Holley w/ mechanical secondaries on a reasonably built 302 Ford. I bought the tuning kit to put the big (50cc)accelerator pump on it and the aggressive pump cam. It ended up running much better with a small pump cam and the small (30cc?) pump diaphram. Too much fuel can act similar to too little.

[McGuire]

Quote

Originally posted by Big Block 8


Otherwise I agree, but I hope anyone won't take that as a hint that 6000 rpm is healthy for a stock Pontiac V8.;)

Excluding perhaps the Ram Air IVs and 73-74 SDs, I personally wouldn't recommend ever going past 5500 rpm with the original rod bolts, as they tend to stretch. I'd go for balancing, ARP bolts and good set of valve springs first.

I totally agree. The Pontiac is a perfectly respectable engine if you respect its qualities, high rpm operation not being one of them. Here I was merely illustrating the best-case airflow requirements. If anything you could say the 670 is a little big, at least on paper. I have seen a lot of 400+ CID V8s run great with a 600 CFM Holley, or the Rottenchester Quadrajet. Some say the Q-jet flows 600 CFM, others say 800 CFM. I say carburetor airflow ratings don't work quite that way.

You sound like a Pontiac guy... me too. Pardon me while I ramble...One of my favorite street engines ever was a 455 I threw together out of junkyard parts years ago. It was a 497 CID stroker created by offset-grinding the crank for Buick rods with OE replacement pistons. CR worked out to 10.1:1 or something. Old Edelbrock dual-plane intake somebody gave me, Q-jet, pocket porting and matching, $50 Melling camshaft with 1.75:1 six-cylinder rocker arms, homemade pushrods, stock distributor and swap meet headers. I didn't have $500 in it, but on the dyno it made 450 hp at 5000 rpm and over 500 lb ft from 2000 to 5000 rpm. The boss came over, took one look and said, "Bullshit. I don't believe it. Get that #@*& junk out of here before it comes apart and makes a mess all over the floor." I put it in my big old two-tone '72 Grand Prix (very sharp, white interior!) and liked to go around picking on the musclecar guys. Big fun... should have stuck it under the bench and kept it, but that engine ended up in a street stocker at the local dirt track...guy won the season championship two years running before it finally blew up for good. Cheater.

[Big Block 8]

Quote

Originally posted by McGuire


I totally agree. The Pontiac is a perfectly respectable engine if you respect its qualities, high rpm operation not being one of them. Here I was merely illustrating the best-case airflow requirements. If anything you could say the 670 is a little big, at least on paper. I have seen a lot of 400+ CID V8s run great with a 600 CFM Holley, or the Rottenchester Quadrajet. Some say the Q-jet flows 600 CFM, others say 800 CFM. I say carburetor airflow ratings don't work quite that way.

I knew what you meant, just thought to voice out in case Menace or someone else with a Pontiac gets too excited.

Most early seventies Q-jets were labeled as 750 cfm, the later ones 800 (or even 820). The 8x0 Qs were still used with the most asthmatic smog strangled 180 hp 400 cids, so one has to wonder if their vacuum secondaries ever opened and what was the actual point of using such big ones. Guess it doesn't hurt, with those small spreadbore primaries.

Rochesters are truly love/hate carbs - some say they're the best and some in turn say they only make good fishnet anchors. Those I've had experience with have worked very well, but I took the advice and never tried to tune one myself. It's a complex thing and easily gets messed up if tampered, but when it works, it's good.

Quote

Originally posted by McGuire

You sound like a Pontiac guy... me too. Pardon me while I ramble...One of my favorite street engines ever was a 455 I threw together out of junkyard parts years ago. It was a 497 CID stroker created by offset-grinding the crank for Buick rods with OE replacement pistons. CR worked out to 10.1:1 or something. Old Edelbrock dual-plane intake somebody gave me, Q-jet, pocket porting and matching, $50 Melling camshaft with 1.75:1 six-cylinder rocker arms, homemade pushrods, stock distributor and swap meet headers. I didn't have $500 in it, but on the dyno it made 450 hp at 5000 rpm and over 500 lb ft from 2000 to 5000 rpm. The boss came over, took one look and said, "Bullshit. I don't believe it. Get that #@*& junk out of here before it comes apart and makes a mess all over the floor." I put it in my big old two-tone '72 Grand Prix (very sharp, white interior!) and liked to go around picking on the musclecar guys. Big fun... should have stuck it under the bench and kept it, but that engine ended up in a street stocker at the local dirt track...guy won the season championship two years running before it finally blew up for good. Cheater.

That's quite an impressive achievement! I guess I really have the softest spot for Pontiacs, although I have had lots of fun with a good few GMs, Mopars, VWs, BMWs, Fords, Porsches, Volvos, Citroens, Mazdas, Toyotas too. My first "real" project was a 76 Firebird with a Pontiac 400. I spent an enthusiastic pile of money on that, although I also made a few bargains from a wrecked one - Edelbrock RPM package, roller rockers, Herb Adams suspension tweaks, reinforced brakes, steering box, 5-speed manual etc. It was worth it in the end though, 445 hp @ 5500 rpm and worked and drove like a charm. I even got most of my money back when I eventually sold it (not counting the personal work hours of course). Other Firebird with a 400 and auto was on budget less than $1000 on engine and suspension, but it still turned out to be around 320 hp (rough ET estimate), with silky idle.

[McGuire]

Quote

Originally posted by Big Block 8


Rochesters are truly love/hate carbs - some say they're the best and some in turn say they only make good fishnet anchors. Those I've had experience with have worked very well, but I took the advice and never tried to tune one myself. It's a complex thing and easily gets messed up if tampered, but when it works, it's good.

I always liked the ol' Rottenchester QueerJet and was very comfortable with it, but as a small cog in the GM evil empire I had the right parts and info all around me. Later on as the stuff became obsolete, the easiest way to get parts was to scrounge the junkyards. The best Q-Jet components (vacuum breaks, secondary metering rods, etc) were to be found on certain Cads, Buicks and Oldsmobiles, and you could buy old carburetors for $20 per 55-gallon drum-full.

Now all that Rochester stuff is pretty hard to come by, but you can get any Holley part you want just by picking up the phone. (Still can't get my brain totally wrapped around buying NEW carburetors, though.) The best thing you can say about the Q-Jet now is they won't stain your intake manifold with weeping gasoline like Holleys invariably do. (I HATE that.) Barry Grant says his carburetors won't, but I haven't tried the Demon myself yet. With EFI taking over the universe, I mess with carburetors less all the time.

[McGuire]

Quote

Originally posted by Big Block 8


That's quite an impressive achievement! I guess I really have the softest spot for Pontiacs, although I have had lots of fun with a good few GMs, Mopars, VWs, BMWs, Fords, Porsches, Volvos, Citroens, Mazdas, Toyotas too.

Of course I would love to say I carefully planned that combination, but essentially it all came together through Pure Dumb Luck. The engine was assembled entirely from found objects, stuff lying around my shop and at work..many of them near the dumpster.

[McGuire]

Quote

Originally posted by McGuire
With the #1 piston at TDC (first cyl on the RIGHT bank on a Pontiac)

Oops, my bad -- I made a big mistake there. On a Pontiac V8 #1 is first on the LEFT (from the driver's seat).

....On Pontiac and Cadillac V8s, contrary to standard GM practice, the frontmost cylinder is on the right bank, as the right bank is offset forward instead of the left (this puts the distributor on the right side of the camshaft, so the drive thrust is upward rather than downward, sparing the oil pump).

However, the cylinder numbering is still conventional GM, with 1-3-5-7 on the left and 2-4-6-8 on the right. Sorry for any confusion...as I get older I am getting soft in the head.