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I have a puzzling problem with a '77 350 Chev engine.

A friend of mine bought a car that has a modded 350 in it which, though it has wonderful power, hesitates off idle when launching it & backfires. It has a high rise manifold, Holley 670 cfm double pumper, headers going into 3" dual exhaust & a mystery cam & runs SUPER rich. I stuck my gas analyzer on it &, if adjusted to run anywhere near an ideal air/fuel ratio, backfires when idling. Right now it's a pig on gas & carbon fouls the plugs after a bit. The previous owner, who can't be reached, used it for drag racing, but replaced the engine with another (supposedly) more streetable engine. It still has a rather lumpy cam & the carb doesn't look all that old. It has to run premium, so I'd think that the compression has been bumped up too. Anyone have experience hopping up a 350?

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Since writing this post BigWilly! may have helped people, but has not within the last four (4) days.
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run, gas, backfires, cam, engine
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20201024 182842
(12 hours after post)
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You need to do the basics first. Tune up is needed, plugs, wires, rotor, cap. Make sure that there is no air leaks from any hoses or gaskets. Check the timing most of the time when it pings its too far advanced also take out plug 1 and set the distributor at plug 1 to make sure that it is at top dead center. Even with the modified cam it should be within +/- 5* from stock settings. You might have to run octane booster in the tank that might stop the ping. This is just the basic things to check there is so many more things that you might have to do to make it run well.

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