After 20 min. Car bcakfires/stalls then will not start

[BruceA]

I have an 72 340  4 bl that I have done a large amount of work on. When I take the car out after 20 minutes or so of good running the car will die when comming to a stop.

After this happens then the car is had to start but will after coaxing. I drive another 5 min or so and then the car starts to backfire and will eventually die. It will start again after a bit but the backfiring gets progressivily worse no matter the load on the engine until it dies again and will not start uthil it has cooled off.

I have replaced the ignition wires, carb, cheacked and replaced the wires and checked the distance of the fuel line from any heat source but can not improve this condition.

I have the brain box a resonable distance away from the exhaust and it does not seem to be getting excessivaly hot. Is it possible that a valve is sticking open after this amount of time or have I overlooked a simple solution>

Perplexed

[MEK-Dangerfield]

Welcome aboard here. 

  My first thought is fuel related. If not vapor lock, then some kind of blockage in the line. Is the fuel filter new? Has this car sat for a long time? Maybe crud, or stale gas is in the gas tank. My line of thought is you are losing fuel pressure. How about the fuel pump, is that new?

  Mike

[ShelbyDogg]

Do you smell lots of gas vapors when you take off the air cleaner?  Could be a sticking float.

[Bullitt-]

Don't overlook electrical, I suppose this is a stock ignition, the ballast resistor could be going bad. In my case the car usually started & ran OK but from time to time it might not start or run poorly, new ballast & things cleared up & ran generally better than before. Also a bad coil can act similar but most of what I've read a longer cool down period is needed.

        Welcome To C-C.com.... 

[MEK-Dangerfield]

He mentioned backfiring, which is why I assumed it was running lean, thus my fuel problem guess. He didn't say if it was backfiring thru the carb(my guess), or thru the exhaust pipes. 


  Mike

[cowboy]

-
No gastank ventilation...

[buzzard]

 Loosen the gas cap and go for a run. If prob. gone replace cap. Or drill a 1/16 hole in it.

[FY1Cuda]

My guess is that it's possibly a bad coil.  If replacing the coil solves the problem, then you need to check if the wiring is correct.  The voltage to the coil should be around 8 volts or something if the ballast resistor is doing its job.  If the ballast resistor is wired incorrectly, the coil is getting twelve volts all the time (not just at starting) and eventually the coil will die.  A burnt out coil will cause backfiring as the engine struggles to run (gasoline is accumulating in some of the cylinders and not being sparked).  Good luck.

[cmesa]

I had the same problem, I replaced the coil, the wires, I even got a brand new carb. I finally fixed the problem by replacing  my timimg chain. My car would run for any given amount of time, then it would not start till it cooled down. Then it started to backfire, after that I gave up and called in a mechanic and he changed the chain and now it runs normal.

[ted]

A BAD timing chain cant do that.

[ChallengerGary]

had the EXACT  same problem with my Pro Street - replaced the ECU and bingo no more problems.

[jeryst]

I'm betting on electrical. Coil, resister, or brain box.

[BruceA]

Thanks for all the suggestions this is what it turned out to be.

It was a combination of bad spark plug wires and due to my sillines I wired the coil to the wrong side of the ballast resistor.

Once I replaced the coil and the wires it went back to being a good running car.
Thanks for the help