340-2 stalls on stopping

Hi there,

my car used to stall after a "cold start" (and cold weather) on first stop. As soon as the engine got real warm, the problem went away (or happend rather seldomly).
Now that I fixed my choke (see different thread), the problem seems to have gotten worse - now its happening on warm/hot engine too.

The rpm on park/neutral is ok (or a bit high for my taste, around 1500 rpm), but as soon as I put the gear to either D or R, the engine rpms start to drop rapidly and most of the times the engine starts to stall. When I try to rev it in time, it sputters and often stalls completely.

What I checked so far:

- spark plugs are all looking good (light grey), so air/fuel mixture seems to be correct
- carb (Rochester 2GC) seems to be ok, no apparent leaks or loose connections (the car is not equipped with power brakes, so vacuum leaks are rather unlikely - the only vacuum lines are for choke heating and distributor)
- air filter looks like new
- choke is working, choke flap is fully open on warm engine
- distributor + cap + spark plug wires are ok (its not the original distirbutor w/ circuit breaker, but an aftermarket "full electronic" part). When I check the ignition timing with vacuum line disconnected, the timing is at 0 marking (of engine timing marker), with connected vacuum line its around 10 (the timing marker of the engine stops at 5, so its just a guess). The manual says timing should be "2,5 b.t.c.", so I guess my timing is 2,5 degrees to late?

Anything else left to check?

Is it ok to just increase the rpms by turning the central "spiral spring loade" screw" of the carb clockwise?

Cheers,
Harry
 
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1500 RPM in Park/Neutral is WAY high. It should be less than 1000. At 1500 RPM, the carburetor is drawing from the main circuit, not the idle system. Sounds like the carburetor is not adjusted correctly. Do you still have points? If so, you must check point dwell before ignition timing. If you are checking ignition timing at 1500 RPM, you likely have some mechanical advance in as well, so your base timing is retarded.

You also should really tell us what year your car is.
 
Make sure the high stall switch is working on the accelerator assembly. It should change to high stall when you let off the gas completely. This prevents creeping at lights or in drive at a standstill. It will add to stalling issues if not working properly. It should idle around 550 RPM in drive (parking brake on) and not gain much going to park. There is a good carburetor adjustment procedure in the 1967 Chassis Service Manual available in the reference section of this site.

Also, the stock distributor begins centrifugal advance between 550 and 750 rpms, and by 1700 rpms the distributor has already advanced 10-14 degrees. You are taking your timing reading after it has advanced and the reading should include no centrifugal advance to be accurate. Your actual timing will be 10-14 retarded from your reading because of this issue. I would get it running at the correct rpm and then set the timing. That may be part of the stall problem as well.
 
@Dr. Frankenbuick

Thanks for all your hints, but the "switch pitch" gear control switches and ignition timing are ok.

I think the idle circuit of the carb is the culprit (as suggested before by others here), as the car does not have a stable idle and I have to increase idle rpms (to avoid stalling) to a point that definately is not being supported by the idle system anymore. So it seems the idle system has a problem - over that the carb is making strange "popping" noises when idling.

Will take the carb apart and see for myself - thank god I already got a carb rebuild kit here.

Cheers,
Harry
 
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It won't be the "idle circuit" that has a problem, yet, anyway. As suggested, your engine is running way too fast to be using it.

As Larry mentioned, 1500 idle is way too high, you are no longer running on your carb idle circuit, and have pulled in way too much mechanical timing. You cannot set your initial timing at that rpm. You should must get your idle down to 6 or 700 rpm to ensure your distributor weights are parking.

Before you pull the carb, maybe you should do a compression test and see that all cylinders seem happy. Of course, always watch for vacuum leaks. You probably have a line going to the automatic transmission. Vacuum leaks can also be a result of gasket failures.

You mention your timing mark stops at 5, that is unusual. I have a 1967 340 here, and it goes to 15.
340_timing.JPG
If for some reason yours does stop at 5, get yourself an advance timing light where you can dial in the advance on the light, so all you need is a "0" mark on your engine.

And, being as someone has replaced your distributor for some reason, could the popping be a crossfire or ground from the ignition wires. Do the touch anything that could ground one, or run tight parallel to each other where they can crossfire? Are they correct HEI wires, should be 9mm?
 
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@Bob

Sorry for the confusion, I guess I left out a few details:

- I was able to check ignition timing and set idle rpms (warm engine, trans in Drive) to about 550
- Even at this point I found out that this idle setting was not stable, after almost every revving up by pulling at the carb lever the rpms would either drop (and the engine start to stall) or at least start to become rough
- So I did a testdrive and the engine stalled on *every* stop, sometimes even while driving real slow. Each time I found it harder to get the engine running again, I had to hit the pedal more and more to get it revving and even be able to put the trans to D to drive on
- After stalling just another time I wanted to increase the idle rpms to be at least able to reach home. So I turned the idle adjustment screw (the one with the spring, sitting on the bottom carb lever) more and more in (clockwise). Each time I started the engine to see if the idle would now at least keep the engine alive
- After finally reaching this point, where the engine stayed on and the rpms where way too high for a normal idle (by feel 1000-1500, did not have my timing light with me) I realized, that the idle adjustment screw was approx "half way in"

So after all this messing around I think that the carb is causing this idle problem, maybe the "idle needle holes" are (temporarily) plugged and so the engine is running on the "off idle discharging holes", because you have to put the "idle" rpms too high to keep the engine running ("off idle operation" on the attached picture).

Any other ideas?

And yes, of course my timing marker stops at 15 too - my bad.

Cheers,
Harry
 

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If you can get it to idle at 550 in gear, but it wont accept any change in throttle position without stopping, I would suspect the accelerator pump in the carb. It sound like the carb needs a rebuild kit and good cleaning anyway.
 
@Dr. Frankenbuick

No, its quite a bit different. I managed to get it to idle at 550 in gear exactly this one time - could have been luck. Next time (after driving, restarting or just after revving) it wont idle anymore and stall. Driving/accellerating however is no problem (with "deadjusted" high rpm idle to keep it running).

So I really think the accelerator pump is ok (or better said not "shot completely"), but the stuff directly affecting idle is not.

Cheers,
Harry
 
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Hi guys,

wanted to share a short summary of my carb rebuild:
Found lots of a dark grey/brownish substance mainly inside the float bowl, when dried it appeared to be a fine powder. Found this stuff in most other parts of the carb, certainly obstructing air/fuel flow. The bottom photo shows the total remnants inside my cleaning bucket, after all the carb cleaner dried away...

I'd suppose this are rust flakes coming from the fuel tank/fuel lines (although the carb is equipped with a rather new looking filter in the inlet).

Apart from that both idle screws were set awkwardly different (2 1/4 turns and 1 1/4 turns), which does make no sense but lets me assume, that one of the idle systems was not working at all (or very lean). I set both to 2 turns and was able to start the engine, adjust idle rpm to keep it running and then tune the idle screws to have best/"smooth" rpms. Ordered a vacuum meter to finetune this even more.

Even now the engine does have a "rock solid" idle (cold and warm), I was easily able to set it to 550 rpm as required by the timing guide/setup manual. The car drives like new :)

Thanks for all the hints and suggestions!

Cheers,
Harry

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Check the throttle shaft for wear in the casting. The Rochester 5 GC is a very basic carburetor and very reliable. I would start with the idle screws set at 1 1/2 turns out, with the idle speed screw backed out, and set the idle screws for the highest vacuum. If your vacuum gauge readings are fluctuating then spray around the carburetor with some either and see if the engine reacts. If it does, then you have a vacuum leak somewhere around or on the carburetor. If the throttle shaft is worn, no amount of adjusting is going to solve that problem. I can recommend a shop that does excellent work bushing the throttle shaft and rebuilding the carburetor. They aren't inexpensive, but the work that he does is worth the extra expense. A lot of carburetor rebuilders say that they can do the work, but I have learned from experience that some of them don't get it right, and the throttle plate hangs up because of misalignment.
 
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