The Stovebolt Site Forums The Shop Area Engine Shop 235 with 1bbl Rochester still stumbles on acceleration

Hi all, a while ago I posted about my 235 with a 1bbl Rochester having a bad stumble and hesitation when the throttle is applied. If I get into it real slow it revs up fine, but if I get into it any quicker it falls on it's face, even dies sometimes. So I finally got the chance to go thru the carb again. I made sure the accelerator pump passageway is clean and it squirts a nice stream of gas when the throttle is applied, but still have same problem. I did notice that the throttle can get about 1/8 a 1/4 open before the accel pump is activated. I'm thinking that could be the issue. Is there a way to take the slop out of the accel pump linkage, or even bend the little link more so the pump comes on sooner? Anyone have experience with this problem?

You found the problem, the pump needs to squirt fuel as soon as it moves. Bend the rod if needed to make it happen. You could have worn holes with worn rods creating the extra movement.

http://www.tocmp.com/manuals/Carbs/Rochester/B/index.php

The above link is parts and instructions for the model B, its pretty small but may help.

Ok. Thanks for your help. I have been running the engine in the garage without an air filter, I'm wondering if the filter would richen it up and help some too. In the mean time I'm gonna try playing with the accel pump linkage.

If the air filter makes it run richer, its either dirty or too small.

Ok. So that shoots that theory. I guess I'll focus more on getting the accel pump to come on quicker.

I shortened the little bent link that connects the accel pump to the throttle plate, approx 1/16" shorter and the engine ran better but not quite like it should. I guess i should try bending it just a tad more and see what happens. It does run best when up to full temp (180) and not as well before that. Perhaps I should adjust the choke a bit richer. Anyone know the best way to set the automatic choke properly?

You mention that it runs best when up to full temp(180).
Is your manifold heater working properly?
This is designed to keep the fuel from falling out of suspension and pooling up on the bottom of the intake runner when the engine in not up to temp (180).
Carl

It pivots nicely, I will have to look and see if the spiral spring moves it as it gets warm the next time I run it. Other than that I don't know what properly would be. I haven't paid any attention to the manifold heater while the engine is running, I'll give it some regard in the future.

The flap should close when the engine is cooling off. On a cold start, check to see where the flap is and if you can move it one way or the other. This flap directs exhaust up and around the base of the carburetor through casting holes in the intake. As the manifolds heat up, the spring relaxes letting the weight on the flap arm pull the flap closed. This then directs the exhaust out the manifold and down the pipe. In your climate, intake manifold heat is very important.

Keep monkeying with the linkage, if you go to far, the engine will let you know.

That annoying just off idle stumble can be hard to pin down. As mentioned above, the accelerator pump must work, right now. Other things to look for include, the "power valve". That lovely long spring loaded bit. It must be free in the the hole. Only vacuum to move it. I "polished" mine with 600 grit paper, and a hand drill. As soon as it was free I stopped. The check valve in the support must fit free as well. Then to finish(still had a stumble sometimes) I added a 1/4 turn(rich) to the air bleed screw(from the book setting) and retarded the ignition just a bit. Those things worked for me. For some reason my truck(55-62 235) pulls a bit less vacuum that most. Close to 15 inches at idle. Steady and working, just less than others I have worked on. I have looked at air leaks, none found so far. Head has been off, and valves checked(good machine shop) and they are good. My gauge on different 235 shows closer to 17-18 inches so not the gauge. Heat riser works as it is supposed to. But with the little adjustments it runs and starts very well. Gas mileage is not too bad(about 16-18) and it runs strong. I have not replaced the Carb, It works so why would I? It does hate dirt. And that has been my only issue. So I have 2 fuel filters, one before the pump, and one before the Carb. They have helped make it reliable and fun.

Climb up so you can see down the throat of the carburetor as you open the throttle slowly to avoid a big squirt from the accelerator pump. If the airhorn is warped, as you slowly open and close the throttle valve, you will see wet gas form and evaporate along the parting line where the gasket is. This will cause a stumble.
Carl