Odd stall problem I need some help with please on my 73 340

[73runner]

Car runs great and starts right up cold in the morning with no problems. I start by giving it gas as per manual .. Twice to the floor (to prime intake and set choke) and hold part way when cranking... Car runs smooth all the way to full warm up some miles later, after it reaches operating temp (180 degrees per thermostat) still not a problem.

However , once it's been at operating temp for a while or after I stop and the engine heat soaks a bit, it will stall if I hit the brakes just before the car stops. rpm drops quickly, vacuum drops, and often as not it will stall or at best run like crap for a few seconds until the problem "clears" and engine goes back to normal.

Engine has about 1900 miles with easy break-in after complete overhaul. Compression at 9.2:1, timing is set at 9 degrees advance, stock thermoquad with 440 main jets. Thermoquad rebuilt with engine exactly to specifications. Distributor and ignition original. Advance (mechanical and vacuum) both appear to be operating correctly. Thermal vacuum and OSAC both bypassed. Vacuum for distributor taken of back of carb. Stock intake and exhaust manifolds.

Any ideas or suggestions welcome.

Thanks in advance for any help

 

[bandit]

Sounds like ignition system problem. Make sure the coil is good. When they start going bad, they will act up when hot and be ok when cold until they finally go. Might be your culprit.

 

[67 coronet]

if i understand your explanation,it trys to die after it gets warm and when you are stopping?could be a float issue.also,check to make sure you are not getting junk into the carb from your evap system.

 

[73runner]

if i understand your explanation,it trys to die after it gets warm and when you are stopping?could be a float issue.also,check to make sure you are not getting junk into the carb from your evap system.

floats set at 1" per spec. from a zip kit I had. Checked booster for leak and found none. Booster holds after shut down and is good for two good pumps after engine is off.

Problem happens just just before the car comes to a stop. If I ease the car to a smoother stop problem does not present itself so perhaps it's the floats or the evap.

 

[72RoadrunnerGTX]

Sounds like a lot of effort to retain stock specifications but you may be running manifold vacuum to the vacuum advance. Try connecting it to the correct timed port it was designed for and reset initial timing.

 

[Hemi Rebel]

Is this a power brake car? Could be a booster issue. A vacuum leak with your foot on the brakes could cause this

 

[73runner]

Sounds like a lot of effort to retain stock specifications but you may be running manifold vacuum to the vacuum advance. Try connecting it to the correct timed port it was designed for and reset initial timing.

Vacuum for distributed taken off back of carburetor to correct port on driver side of centerline. The other port on the right is for the choke pull off.

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Is this a power brake car? Could be a booster issue. A vacuum leak with your foot on the brakes could cause this

Yes. Car has power brakes. See above.

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Thanks to all so far who've helped with their advice..... Much appreciated.

Problem is less severe or not present with A/C off. I know that compressor head pressure increases when car is fully warmed up.

Checked timing to verify and it is between 9 and 9-1/2 degrees advanced.

Checking the bowl vent which may not be opening enough at running idle warm after the choke fully disengages.

Other things to check:

Idle led step up solenoid for a/c.

Recheck vacuum advance hot... May be loosing some vacuum when fully warmed up...this is going to hurt.

Coil and all ignition connections..... Tachometer needle on dash and electronic timing light tachometer both show variation in rpm... May be a loose or dirty connection somewhere....

 

[72RoadrunnerGTX]

Vacuum for distributed taken off back of carburetor to correct port on driver side of centerline. The other port on the right is for the choke pull off.

Any port on the rear of the Thermoquad is manifold vacuum, as required by the choke pull off, to be clear the correct vacuum advance port is located on the front of the carb, driver's side of center line IIRC. Should not have vacuum present, no advance at idle per factory specs.

 

[khryslerkid]

It's been awhile, does your throttle still have a "anti-stall" dashpot? A small solenoid in front of your throttle leaver on the side of your carb. Some are electric, some are vacuum operated. It's a fast idle control to keep you car from stalling during sudden stops. They can be adjusted or replaced if bad.

Just read that you have to check this yet. It use to be a common prob back in the day when I worked on these.

 

[73runner]

It's been awhile, does your throttle still have a "anti-stall" dashpot? A small solenoid in front of your throttle leaver on the side of your carb. Some are electric, some are vacuum operated. It's a fast idle control to keep you car from stalling during sudden stops. They can be adjusted or replaced if bad.

Just read that you have to check this yet. It use to be a common prob back in the day when I worked on these.

No anti stall dash pot or solenoid. Just the choke pull off dash pot. Don't even know what they look like.... None I can find in the FSM either... Do you have a picture?

 

[khryslerkid]

http://www.allpar.com/forums/topic/35364-tuningtiming-the-318/

This is just a standard pic, not mopar... Also a link on allpar talking about one on a 318

Damn! Sorry about the size...

 

[idrivemopar]

I am curious, as I have 340, it has the bigger Thermoquad, but I am also around 9.3 for compression with a Crane 272 cam, and my timing is fully mechanical, no vacuum advance with initial timing set at 17 degrees before, and although I have not had her out yet for a drive, seems to run good. If you have timing tape on your harmonic balancer, check and see what your full advance is on your car with and with out vacuum advance hooked up, and when its at temperature. Possible that you vacuum advance on your distributor is not releasing when you come to stop and lower idle. Also do you have the correct accelerator return spring installed?

 

[furyus]

Isn't 9-91/2* a little low for timing? Do you have an aftermarket cam in it? They usually need more timing. I would try advancing the timing a little first. It's easy to try, then go on to the harder stuff..........

 

[73runner]

Spring is correct.

Ill check the timing and let you know. Stock is 5 degrees advance.

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Isn't 9-91/2* a little low for timing? Do you have an aftermarket cam in it? They usually need more timing. I would try advancing the timing a little first. It's easy to try, then go on to the harder stuff..........

thanks..... I will try to advance it a bit. I had dropped it to the current 9-1/2 to avoid some midrange light throttle pinging ....but that was before I changed the main jets rebuilding the carb. May have to use a knock inhibitor.....