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Carb vibration insulators


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#1 ScottNC

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Posted 27 January 2008 - 16:17

At one time there was someone hawking a set of rubber washers and gasket for your typical Holley 4150. They were supposed to insulate the carb from engine vibrations. Anyone know if they're still available? For those who have dabbled in it, what durometer and what type rubber might be suitable if you were going to make your own?

Not that this is the reason for my query but it is related. Last year a local Pro-Mod (big-inch, 3stage nitrous version, not blown alky) had repeated piston burning problems. We did some head repair for him and in explaining his troubles he mentioned he'd installed a new crank just prior to the melt-downs. Never had balanced the new crank and yes it did shake a bit. Long story short, after balancing the crank the piston burning stopped, or at least returned to normal.

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#2 McGuire

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Posted 27 January 2008 - 21:40

The deal you are describing is/was offered by Moroso. Two varieties, four little holes or one big hole. It requires countersinking the carb mounting holes to form recesses for the rubber isolators, eww. Also included a three-layer sandwich gasket with a stainless plate in the middle. Never used one.

Mr. Gasket also offers a vibration isolator, this one being a base gasket of closed-cell foam around 3/8" thick. Never used them either. I am a fan of heat insulators more than vibration isolators, so I like the big thick hardwood/phenolic insulators from Moroso etc. I like four holes for a nice tight carb signal. IMO if the engine is running halfway right, use studs rather then bolts to secure the carb, and use an insulator, you shouldn't have any trouble.

Not that vibration cannot be a problem with float level and operation... in the '80s many of the last of the carbureted four-cylinders (rough runners, mainly due to their primitive emissions systems) used carb vibration isolators...these were a big rubber biscuit around 35mm thick with a steel plate bonded to the top and bottom... constructed like an engine mount in other words. Very often incorporated an electric heater grid for EFE. Not very robust, the rubber tended to rot and create giant vacuum leaks. You could grab the entire air filter assembly with the engine running and by pushing or pulling on it make the engine quit or smooth out. Whee.

#3 Greg Locock

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Posted 27 January 2008 - 22:30

If you are tempted to roll your own you need to use a fuel resistant grade of polymer. The easiest option is probably silicon rubber sheet. Bear in mind that for true isolation it is not simply a gasket of rubber, the bolt must be isolated as a transmission path as well.

#4 McGuire

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Posted 27 January 2008 - 22:42

Exactly. To get true isolation you need two sets of fasteners: one set attaching the isolator to the manifold, and the other attaching the carburetor to the isolator.

#5 Joe Bosworth

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Posted 31 January 2008 - 01:17

May I suggest two more options.

First option: I used to make some pretty affective vibration isolators using wood or phenolic blocks. Drill/bore your holes as desired. Make shallow o-ring locating grooves around bores top and bottom. The larger the o-rings the better. Install with o-rings in locating grooves but allowing the o-rings to partially compress against the flats of carb and manifold. Use your favourite thread locking stuff on the fixing nuts so the carby doesn't fall off over time.

In the real past I simply used an o-ring and spring/nut fixing on Amals and on two cylinder aircooled things as well as four cylinder water cooled, but the block with o-ring grooves is better. Just remember your gains are from dampening the worst vibrations, you can't eliminate the vibrations no matter what you do,

Second option: Buy an already developed one. If it were me at this point in my life and with such things available over the counter or via e-commerce I would spend the +/- $250 as option 1 costs money and time as well. I recommend contacting Kostecki Motor Sport who have developed a dyno proven "Vibration Dampening Carb Spacer" as they call it. Look them up at www.kec.com.au . Most carb choices are covered.

They can also be reached via phone, 61 89 209 1222 in Perth, Western Australia. Don't forget the time change differences. If fax is easier try 61 89 209 3736.

They have some distributors/dealers in the US but KEC can advise as to who they are.

Dyno tests have shown substantial HP gains. The gains to be had are highly dependant on your situation.

Regards

#6 cheapracer

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Posted 02 February 2008 - 13:49

Originally posted by Joe Bosworth

In the real past I simply used an o-ring and spring/nut fixing on Amals.

Regards


Thanks for the bad dreams I will have tonight (he says as he shudders).

3 years working on Trumpys, Beeza's and Nortons does that to you.

By the way Scott, as a rule of the thumb the thicker the spacer/insulater/riser you use on a V8 (increasing your plenum volume), you raise the torque peak.

#7 carlosfierros

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Posted 06 February 2008 - 04:11

I solved the problem when my carb vibrates, in this way.
1. I put larger threads (bolts).
2. Used a elastic gasket about 10mm thick between the carb and the manifold, synthetic rubber.
3. I used rubber washers under the washers made from cutting a hose
4 adjust the nuts not much to let the carb move a little, and then lock them, to avoid them loosening, for exam wiring them.
5.be happy :clap:

best wishes.
Carlos.
Race Car R&D engineer
Maldonado Fierros Engineering