Carb backfire?

[Dave22443]

OK, this one has me stumped and I know some of you can offer an explination.  Here's the problem.

First off, the specs on the engine.
1969 Plymouth 383 Super Commando with 727.  All stock except headers. Milage unknown but believed to be "high".  I.e., the engine is a little tired.  Fuel used is 92 or 93 octain unleaded pump gas with no additives.

Engine starts and runs fine under normal driving conditions.  Sometimes there is a little smoke when it first starts, but nothing major and it clears up quickly.  (Suspect valve seals)  I don't have to add oil between changes so its not excessive at all.

With the engine at full operating temperature and driving normally, it will, on occasion, backfire through the carb when I mash down on it.  Not every time, but maybe once every ten times perhaps?  This can happen from a standing start, or from 60 mph when passing someone.  But again, only once in a while.

It has good wires points and plugs.  The original carb was rebuilt and seems to be working fine.  The rebuild guy told me he believed it was 525 CFM.  The timing is set at 5 degrees.  I tried swapping the carb with a friends loaner Eldelbrock 600 CFM, and although the car does respond a little better, the occasional backfire through the carb did not go away.  I have tried playing with the timing but the engine really seems to run best at 5 degrees BTDC.  The engine really does run quite well besides this.

Not sure if this is enough information so ask if its not.

Any of you gear heads out there got any ideas of what might be wrong with it?  I'm planning to swap it out with a 1974 440 I'm having rebuilt in the spring but the 383 will go into my sons car afterwards so I would kind of like to know how to fix it.  A full rebuild for the 383 is at least 2 years away so I hope its nothing serious.

Thanks for the help!

-Dave

[Lord Warlock]

I was under the impression that the factory magnum/super commando motors had a 625cfm carter avs carburator on them.  I had a stock 440 magnum motor, that was replaced with a 383 magnum motor for a while, and both the stock carbs were the same avs. 

The only times I've had it backfire thru the carb was when I had the distributor advanced a little too far (vacuum advance) It usually ran great, but it took longer to crank/start.  I did have a problem with one of the floats in the carb that started filling up with gas and caused the backfire situation, it stopped when i rebuilt the carb and changed the floats and backed timing back a bit. 

The 383 magnum I had had been totally rebuilt with racing pistons/rods and ran just as fast as the stock 440 did up to a top speed of 120, but that may have been the cam or some other build issue that I wasn't aware of. (I didn't build it, just bought it that way at a salvage yard and swapped it for the 440 when I spun a bearing in the original motor) Wish I never threw away the factory block though, now i regret not having the stock block, but back then I was young and stupid.

[Chryco Psycho]

Most backfires through the carb are the result of a lean condition , you may need a larger squirter

[darrin75]

Did not want to start a new thread, but what can cause a backfire through the exhaust after getting on it and letting off the gas.  Too Rich??

[oldkimmer]

Ur dizzy will b wore out, with the shaft moving side to side, especially when standing on it .......take the cap off and see if u can wiggle the shaft to and fro...........ur timing chain will also b stretched creating a lean condition.........kim.........

[darrin75]

Ur dizzy will b wore out, with the shaft moving side to side, especially when standing on it .......take the cap off and see if u can wiggle the shaft to and fro...........ur timing chain will also b stretched creating a lean condition.........kim.........

Say What???

[Dave22443]

I think he's trying to say that the distributer might be worn out.   

[darrin75]

unlikely ...Its New