The Stovebolt Site Forums The Shop Area Engine Shop 235 stalls and stumbles off idle

This engine is a 1957 Chevy 235

Does anyone know common problems related to off idle stumble and stalling?

-New rebuilt Rochester carburetor.
-Engine starts and idles pretty smooth.
-When I snap the throttle, or open it relatively fast, it starts to stall and acts as if it is not getting enough fuel. This is consistent cold or hot.
-The fuel pump is pumping plenty of fuel.
-Checked timing, I'm timed a little advanced (using the bee-bee). It still can be seen in the window.
-When revving engine it misfires.
-Engine ran normally prior to installing professionally rebuilt carb.
-The vacuum advance seems to be working normally. Although it moves to full advance as soon as I snap the throttle before the engine has a chance to rev.

Any ideas on what the problem might be.

It sounds like the accelerator pump isn't working. With the engine off,have you looked down the carb and opened the throttle?

Yes, there is a healthy squirt of fuel so the pump seems to be working. I'll double check.

try disconnecting the vacuum advance line and plugging it. See if anything changes. Did you used a new gasket under the carb? Same as the old one?

Okay, confirmed accelerator pump works.

Disconnected the vacuum advance and symptom gets much worse. Engine stalls with throttle open, which makes sense.

Checked timing again and set to zero advance. Engine sputters and just about stalls with throttle open. If opened slowly its more well behaved but not perfect.

Checked vacuum at advance connection (10-15 inches).

Engine vacuum is in the green (I forget the value) and starts/idles well.

if it ran normal before installing the carb, why did you need to replace the carb

It was a old rebuild and the accelerator pump wasn't helping with starting. I replaced it (the accelerator pump) but still wasn't getting cold start fuel help so I decided to pay for a pro-built carb.

Looky here boys, whatcha think?


ALSO: with a clear, clean mind...recheck the firing order on Dizzy....carefully. If that's OK, rotate Dizzy while running, each direction, and see if it will rev. It probably wants advance to 5 deg from BB ball. (pointer on triangle with timing light?). If no result: Be sure choke it operating. Then try revving with some choke on and see if it revs. These trials may give clues.

If the firing order check doesn't fix this, t have to offer my same old advice, Get rid of the rochester and replace it with a Carter YF.

Just throwing this out because I've heard it many times on here. Is this the carb that has the extra hole/slot in the base plate gasket? I'm not the carbking so I'm just asking.

Thanks fellas, I'll have more time this evening to play around. Maybe I'll make a short video to illustrate the symptom better. I have to agree with MikeE, these Rochesters are a PITA. I've had nothing but problems with them on my 216 and now 235. My dream was to slowly fix things on the 3600 as I drove it here and there for fun. The reality is that I spend all my time trying to troubleshoot runnability issues with the Rochesters.

ok, I had mine rebuilt by recarbo got it back installed it, cranked right up.. did nothing to anything...

neat truck you got

Nothing wrong with a Rochester carb. If it is "professionally rebuilt" correctly, should be nothing wrong with the carb. I did not mention the carb because you said rebuilt so I was thinking spark/timing but it does seem like it could be a carb issue.
What exactly do you mean by misfire?
1. Running rough?
2. Backfire at carb?
3. Backfire at tail pipe?
4. missing? as in; one cylinder not firing? Did you do a spark check on each plug?
5. ALSO as Martin said. VERY GOOD point. There is a hole in the B1 carb mounting flange that must not be covered by the base gasket. Some gaskets have the slot, some do not. There must be a passage slot cut into the gasket from the hole into throat, for vacuum. CHECK THAT ASAP!
If carb has no hole in bottom, disregard.

You might give Mike a call at thecarbshop.com. I rebuilt my Rochester 1brl and had idle problems. Disassembled it and found the main checkball stuck inside the spring. Reassembled and now the truck runs great.

I just compared both carburetors and they appear identical except:

The original one is a Rochester C (one hole in base)
The rebuilt one is a Rochester RP (two holes in base)

I did compare the bases and noticed the extra hole in the base of the Rochester RP. I swapped the bases and tested runnablility, the same symptom is present.

The gasket I'm using between the intake and the carb base has space for both 1 and 2 hole bases.

The intake manifold looks like it is built for the (one hole) carb

** I bought the rebuild from Mike, I'm just hoping I ordered the correct carb **

Here's a quick video:

Start with the basics,I had a simular problem with mine . I had not run the truck for a few years and it was running rough.so I changed the points and the newer style points seemed to have a extra metal spring with I thougot was for other applications as there was a brass spring already part of the points .after install the truck ran great. StarTed to drive and would backfire and miss seemed starved for fuel. Left the truck until the next year,tried carb rebuild coil and fuel pump,was told by a old time mechanic to start with the basics .that extra metal piece from the points was the problem as the truck reved up the points would bounce and needed the extra spring to put pressure on the points to snap it back as the rotor turned .was a silly problem that drove me crazy.but I learned from the old timer to start with the basics.

Well, the 235 is running again! I'm not quite sure what the solution was. I rebuilt the old carb, adjusted the points and added 5 deg timing. It still hesitates if I snap the throttle but runs down the road fine which works for me.

Thanks for all the tips y'all! I love how hopeful this community is when I run into trouble.

These 235s have an off-idle stumble that is hard to totally eliminate. I have a Carter YF on it and adjusted everything and its pretty decent now, but still there.

never heard an engine sound like that, like it was sucking for air,
glad you are some what satisfied with it now, just dont hit passing gear..lol,

That is the trouble with "fixing" too many things at once.........makes determining the root cause of the original problem nearly impossible.