I have a '69 340 Swinger with an automatic, P/S, PDB, and A/C. I have not been in the engine since I have owned it but I believe it is stock from listening to it run, and I will say it is one of the strongest 340's I have ever driven. A problem that I have always had is a "stumble" when taking off at a complete stop, i.e. stoplight and normal acceleration. The carburetor (correct Carter AVS) has been rebuilt by one of the best in the country. It was rebuilt, all parts replated, and mounted for a test run. It still had a stumble so I took it off again and had an old friend of mine go through it which he did and found no problems. He was the "tune-up and carburetor specialist" at the Plymouth dealership where he worked for many years through the 60's and 70's. I put it back on and still a stumble. I have put fresh plugs in it, always run premium gas (ha), checked my timing (stock), good plug wires, clean air filter, and so on. Still a stumble. Any suggestions?? What would you guys suggest for my timing advance? Could I have a vacuum (intake) leak?
Thanks

I had the exact same issue with my 440. I played around with the accelerator pump and that helped a little but the problem didn't go away until I had someone re-jet the carb. It was set too lean from the rebuild for my engine.

1st I'd check that the accel pump shot tip in is immediate when the throttle just moves and that it has an adequate squirt volume & duration. What is the initial set at? Vac can on ported vacuum I am assuming. Is that hesitation from takeoff at WOT or part throttle? With 2 good guys having been inside it I'm assuming the float level is correct. You might bump the initial up to 15 & block the metering rods up & see how it acts. You can easily return those two changes back to stock & do each test seperately & reset it back then do the other test. Yes it might possibly be a vac leak. OE AVS's are lean so is it a pronounced bog or just a lack of power and is it part or WOT? What's your idle speed in drive? EDIT you might check the vac pod for a leak & could clamp the power brake hose shut with vice grips with a rag to protect the hose from the serrated edges & see if the hesitation is relieved. As you know you'd lose most of your brakes so try it in a safe area & you'll know immediately if that is it (PB booster leaking)

there are three circuits that can cause this,

1. Accelerator pump. A problem with it will happen as soon as the the gas pedal is moved, IE: Immediate stumble followed by acceleration as the next circuit takes over. the problem will be more noticeable under load

2. Off idle transfer. the circuits purpose is to smooth the transition between the power enrichment and main circuit when the pump shot dies off A problem this circuit will be characterized by the motor momentarily responding when the pedal is moved ( pump shot)under load and then stumbling before it moves onto the power enrichment or main circuit.

3. Power enrichment. this circuit follows the off idle transfer to smooth the transition onto the main circuit. A problem in this circuit would be also noticeable when cruising at highway speeds and accelerating lightly.

Knowing how the motor is responding and WHEN it is stumbling is the key to identifying the root cause. hope this helped

My guess (key word "guess" not being there) would be that you are focusing on the wrong part. I'd bet it's vacuum leak or ignition related.

I would start with the timing first, too. Throw more timing at it in 2* or 3* steps till the mid 40s total. If that does not improve performance, put it back where you started and go on to something else.

Timing is about the easiest thing to start with, so..........

I agree with you on "what should be". But I just went through this very issue a while back. And it could even have been incorrect timing marks, but I had a mild 360 that did not respond to more fuel nearly as well as it responded to more timing. Ultimately, about 44*.

I was just suggesting a very simple test to, at least, eliminate a potential issue.

It would run alot better and idle cooler with 14-18 degrees of initial timing.