fiona72

Hello,

Thanks for any input you can give... I have a 72 Cutlass with a 1978 350 rocket in it. I have had it over at the shop recently and had the carb rebuilt, new gas tank and a tune up... Why is it still popping through the carb under heavy acceleration?

Thanks,

Greg
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Oldsguy

Welcome to CO.

stan 65 cutlass

you may have a burnt valve?

bkeese

Hold a dollar bill over the exhaust pipe and see if it gets sucked up against the pipe, like closing a flapper valve. This sucking the dollar bill indicates a burnt valve.

Rickman48

Backfire through the carb indicates a burnt INTAKE valve, too much advance in the distributor, or mixed-up spark plug wires.
A broken intake valve spring is also a cause, as the valve's not closing!
I'd check the wires, timing, and try to determine the offending cylinder - pull the valve cover and do an inspection on that side - should be obvious.
Unless of course the valve seals are bad, and you're fouling plugs w/oil!
Look for black, misfiring spark plugs.

bulldog

Could also be a bad camshaft,try running a compression test.

Rickman48

A bad cam won't backfire, as the valve isn't opening all the way!
A bad seat seal - bent or burnt valve, timing way off, or a switched plug wire, will backfire through the carb, as the INTAKE valve isn't sealed when the plug fires!
Bad exhaust valve will backfire through the exhaust.
Keep your air filter on, as I've seen fires in the carb happen!
Compression test will determine a bad valve, though.

oldstrio

A bad cam will backfire through the intake if the exhaust lobe is gone once it fires it has to go some place and when the intake starts to open on the overlap it pops through the carb. typical flat cam syndrome under hard acceleration.

oldstrio

Pull the valve cover and check the valve opening and closing that will tell ya if its the cam or not.

fiona72

Thanks Rickman and Oldstrio,

I double checked the plug wires and they are good, I am going to play with the timing tomorrow. If that don't work, I am pulling the valve covers for an inspection.

Greg