The Stovebolt Site Forums The Shop Area Engine Shop Truck dies at stop.

I have a "55" 235 with dual carbs using W-1 684S. I set the timing using a vacuum gauge by advancing the distributor until the vacuum reached the highest point and backed it off 1". The vacuum is 18". The carbs are balanced and the truck runs great except for one problem. When I come to a quick stop from say 30 mph the engine want to die. It will start right up and idle fine. If I come to a slower rolling stop it continues to idle ok. I recently adjusted the floats on the carbs due to gas leaking when the engine was first shut down and causing flooding. Float adjustment so far has stopped that problem but now I have this engine wanting to die at a stop. Both carbs were rebuilt. 95% of the time the engine runs great. Where do I start to look for a cure for the problem?

Bart: I had a very similar issue with my truck. 55 235 with stock intake and carb. Fortunately I have a Glass filter in the fuel line on the line from the tank to the pump. Sediment would slosh up and cover the line to the pump opening. When I came to a complete stop it would settle back and I was fine. Now I just clean the filter regularly and all is fine. I have another in the line to the carb and it is still clean so the first one is doing it's job. My fuel pump is happy as well. If the float is set too low I run out lean at high RPMs. So I don't think that is the issue. You might see that the wires to the coil cannot move into something that will short them under strong deceleration. Let us know what you find.

I have a glass top filter on the fuel pump and can see that it is clean. I have a new fuel tank and all new lines to the carbs so dirt shouldnot be the problem. If I catch the engine starting to sputter and give it gas the engine will stay running. This might just be something I will need to put up with as a cost for running dual carbs.

obviously a float problem, try tweaking them a bit - might be they flooded at a higher setting because the carbs are too hot and fuel boils when you shut down?

Bill

Dual carbs are a blast. I had a similar issue and it was the floats set too high. I went through plugs fast also. It was super rich but I figured it was better than lean and burning the heck outta my pistons and valves.

I get why we go with spit exhaust manifold. But I don't understand why we need two carbs. Are we drag racing or just trying to get as much power out of these little engines as humanly possible because we can? I am not confident that I can safely bring my AD to a safe stop with a rebuilt stock 235. Any more speed and I'm a threat to society. One thing that may be happening to you on a sudden stop, with twice as much carburetion as what the engine was designed for, is non-atomized fuel pooling up in the intake as the throttle valves (plural) are suddenly slammed shut, cutting off the air supply needed to atomize the fuel (liquid fuel won't combust). The intake ports on the head may not be large enough to keep the fuel atomized under the condition which you describe. Do you have a split exhaust? A less restricted exhaust should increase fuel atomization. Another thing to try is turn the idle screws in a tad. The idle circuit takes over when the throttle plates are closed. Too rich and you have raw fuel pooling up in the intake. With two carbs you double the effect. I am not trying to talk you out of dual carbs, they do look cool. I secretly want to drop a huge, high output Harley motor in one of these trucks just because I like the sound. Well, I guess the secret is out now. Don't judge me, I'm sick.

Are you describing cavitation in the cylinder head?

I do have split exhaust and no, I am not trying to get any more power out of this engine and seldom drive it over 50 mph. I just think dual carbs look cool, more so the carter W-1s. Like this. I will try to adjust the idle screws in some but when set the screws with my vacuum gauge I screwed them out just until I saw the gauge start to jump slightly. The intake is heated with exhaust gas. This engine runs perfect 98% of the time.

OK, I just saw your picture of the two carbs. You are correct, they look too cool to change back to single carb. A guy inadvertently posted a great response to fixing your problem in the harmonic balancer post above your post. It refers to the use of a dash pot to slow down the close of the throttle valve on sudden stops to prevent stalling. I believe they were used on automatic transmission cars in the 70s.

adjust the idle

Slowing down the throttle close makes sense to me. I will have to figure that one out. I am working on possibly a cable accelerator in place of the linkage rods and pivots I build as the pedal is just not as smoth as I would like. I can figure out something to slow that last 1/8" to 1/4" of travel on close.
Thanks

There was a part on those '70s carbs called a "dashpot" that slowed the close to prevent high vacuum and rich mixture under trailing throttle. Part of the early emissions controls. Should be able to adapt to your needs. All mechanical and with a bracket. I don't think they are expensive. Just went between the linkage and the stop. Or close to there.
Here is one just to look at. http://www.autopartswarehouse.com/sku/Univ/Holley/Carburetor_Dash_Pot/H19114.html
Might be what you need.

Copied from incorrect location.

For what it is worth, this problem was very common on American cars around the early 70's. I can't remember what the root problem was, but most companies added "anti-stall dashpots" to the carbs. They just slowed down the last 1/4" of throttle travel. Without them the throttle would slap shut and the car would stall. With them the engine took 1-2 seconds longer to reach idle when you took your foot off the gas quickly and everything was fine.

You can probably find many versions of them in the junkyard. I don't think that they go bad. The ones that I have seen are threaded into the bracket and are adjusted by turning the in or out. Shouldn't be hard to make it work.