6 replies to this topic

[jluetjen]

I've been reading Ludvigson's "V12 Automobile Engines" book and noticed that for years early in the auto era, cars all seemed to have updraft carbs, even to the point of having the carb attached to the bottom of a Y shaped manifold in the center of a V12's, with each branch curving back down to flow into the heads. Admittedly, I'm looking at this with a huge case of 20/20 hindsight, but why weren't downdraft or sidedraft carbs adopted earlier. I can only imagine that there must have been some technological issues that prevented it. But I can't figure out how having the air flow up would be a simplier technology rather then have it flow down.

Any ideas?

[rhm]

Maybe they had trouble with controlling fuel-flow to the jets and were worried about hydraulicing the engine by leaking unevaporated fuel into the cylinders. With an updraft arrangement the excess fuel would just fall into the manifold. Just a guess of course.

[Stian1979]

If I remember corectly the updraft carbs was constrcted on the principle off evaporation.
The inlet air was flowing over a bath off gasoline instead off using a venturie.

Why they did this?
Maybe nobody had thought off the idea off a venturie?
Maybe they had problems construction off one that was eacurate enough?

I think that someone was using a impeler that was splashing in the fuel to make a more fuel ritch air mixture

[McGuire]

Updraft, downdraft and sidedraft carbs all work exactly the same. With an updraft carb the air-fuel mixture must take one additional right-angle turn before arriving at the cylinder; that's the only difference. Given the primitive state of airflow development at the time, that was not a big deal.

Meanwhile, with the updraft there is no chance of flooding if a float needle or other component leaks...the excess fuel merely runs out on the ground. Fuel was dirty and the seals were all metal-to-metal so float valves were constantly sticking.

[Terry Walker]

That's the answer. I remember the Ford A updraught, held together with a single bolt. It was perfectly conventional otherwise - a float and jet job. And yes, unfloodable. This was probably very important in the days of gravity feed, either direct from the tank, like the Ford A, or through an Autovac or similar which lifted fuel into a small tank on the firewall from where is flowed into the carb by gravity. A leaky float needle could empty the entire tank into the engine if the carby was downdraught. Many of those cars also had on-off fuel taps too. My Dad always turned off the fuel tap on his Ford A when he switched off.

[McGuire]

Originally posted by Terry Walker
That's the answer. I remember the Ford A updraught, held together with a single bolt. It was perfectly conventional otherwise - a float and jet job. And yes, unfloodable. This was probably very important in the days of gravity feed, either direct from the tank, like the Ford A, or through an Autovac or similar which lifted fuel into a small tank on the firewall from where is flowed into the carb by gravity. A leaky float needle could empty the entire tank into the engine if the carby was downdraught. Many of those cars also had on-off fuel taps too. My Dad always turned off the fuel tap on his Ford A when he switched off.

Here is the Zenith Model B carb (Model B Ford), externally virtually identical to the Zenith Model A used on the Model A Ford. Larger venturi, improved high-speed circuit. As you say, if the fuel valve is left on and the float valve leaks, the next morning you may find the entire contents of the fuel tank (in the cowl, gravity feed) in a puddle on the ground.

[AS110]

They were often tall engines too,so it was easy to package an updraught carb.I used to see updraughts on industrial engines when I worked on them,also made a compact package ...I still have an old updraught in my carb collection,guess I'll never use it now.Commer COE trucks used an updraught once too - I remember the drivers always complained of hesitation on take off,and you had to put something out of adjustment to combat it,like a rich idle,or advance the igntion....then they'd be back complaining about that,a juggle act.I didn't like the updraught....