Backfiring issue

[emcginty]

I just recently bought a 68 charger with a 383 engine in it.  The owner of the car claims the engine was beefed up to a stroker.  But my issue with the car is at highway speeds I accelerate then back off the engine back fires.  It does not do this at city street speeds.  I am thinking due to lack of backfiring at the lower speeds the timing does not seem to be the issue.  I have not put a timing light on it tho.  What is the local communities ideas on this.  I am not a skilled mechanic so dont beat me up to bad! HAHA

[elacruze]

I'm betting on a too-rich idle mixture and a leaking exhaust near the engine.

[emcginty]

That was my first idea.  The second is water in the fuel...but this seems unlikely due to the lack of consistantancy of the issue at hand.  But Thank you for confirming my idea.

[FLG]

How does she run besides that issue?

Im also thinking a rich condition with a leaky exhaust.

When you back of do you mean you get off the accelerator completely?

[emcginty]

I drove her today and I can not hear any leaks from the exhaust system...I raised the hood about a week ago to listen and did not hear anything out of the ordinary.  When I back off normal acceleration I get what I call "engine breaking" I dont know the official terminalogy with no backfires.  Its only after hard acceleration above 55mph I get the backfiring.  So I would think she is running a tad rich but I have no idea since it does not happen at lower speeds.

[FLG]

Pull a few plugs and see what they look like.

[emcginty]

Ok I will do that next weekend...and let yall know what I find.  Thanks for the insight.

[emcginty]

Ok I pulled a plug today and it had scorch marks on one side...I gaped it and it was .039.  According to the info I have I should be at .035 on this engine.  So I am pulling them all and replacing them with new correctly gapped plugs.  I also pulled the Distributer cap to take a look at the rotary button and of course its messed up too.  So I will be replacing the cap and rotary button.  Any other ideas on what else to check while I am under the hood today and tomorrow.

[emcginty]

I finished pulling the plugs and found a strange thing...The guy who I bought it from put a lawn mower plug in it! He had spacing issues with the headers and was apparently too lazy or inept to do it right.  So hopefully that was the majority of the issues I was having.

[charge69]

Sounds like you have the backfirirg issue figured out. Go ahead and check the timing, initial and at about 2500rpm and see where you are.  The motor should like anywhere from 32 to 36 degrees of timing and be full-in by 2500rpm. this is all done with the vacuum advance blocked off.

Let us know if you can.

Carl

[emcginty]

Well the carb still seems to be running crappy.  So that is the next issue to tackle.  Its a Holley 650 w/electric choke (thats about the extent I have looked at) Due to the spark plugs I spent zero time on that.  But I will check the timing when I pull it back in to the shop.