big bang chevy.!!

[gonzo]

Can someone please tel me why my Chevy 350. backfires when I switch off ? It starts fine and ticks over and runs O.K. but as soon as I turn it off BANG from both side pipes!! I am running a four barrel Holly and M.S.D. A6L ignition setup so is it timing or what??

Cheers GONZO. < the puzzled!>

[chiffer]

Originally Posted by gonzo

Can someone please tel me why my Chevy 350. backfires when I switch off ? It starts fine and ticks over and runs O.K. but as soon as I turn it off BANG from both side pipes!! I am running a four barrel Holly and M.S.D. A6L ignition setup so is it timing or what??

Cheers GONZO. < the puzzled!>

Ha Ha, should have put a Ford engine in instead. Sorry Mike, only joking. Could be pre ignition. Excess fuel left in the manifold and then re-ignited.
Check the timing as well.

Hope this helps

[wilf]

Could be over-rich idle mixture, or too retarded ignition, or air leaks into the exhaust manifold/slip joints, or even incorrect valve clearance adjustment holding the valves open a tiny amount when they should be closed. Lots of possible causes.

Out of interest, does it do it after it has only been running for a short while, or only on a fully warmed through/hot engine?

[Alex Miller]

Probably running a bit lean, or the cam timing slightly amiss.

Backfire is caused more by unused oxygen in the exhaust gases than unburnt fuel, as there will alays be some amount of unburnt fuel in there.

If the mixutre is rich, there won't be any oxygen in the exhaust gases to ignite the HC's.

[Alex Miller]

Probably running a bit lean, or the cam timing slightly amiss.

Backfire is caused more by unused oxygen in the exhaust gases than unburnt fuel, as there will alays be some amount of unburnt fuel in there.

If the mixutre is rich, there won't be any oxygen in the exhaust gases to ignite the HC's.

[wilf]

Originally Posted by Alex Miller

If the mixutre is rich, there won't be any oxygen in the exhaust gases to ignite the HC's.

There would be if there are any air leaks in the exhaust system........but something causing the exhaust manifold to overheat in the first place (like retarded timing) should be the first area to eliminate, I reckon.

[Alex Miller]

Good point, a leak in the exhaust would do the same thing as a lean mixture.

I'd check the timing and the mixture, the answer almost certainly lies somewhere in there.

[gonzo]

Thanks guys I think it must be timing as it didnt do it on my old dizzy and coil setup and yes it doese it on cold or hot engine and the carb setup is still the same as before. think I may have to spend some money on a proper tune up. manifolds do not appear to leak and I dont have any other places for air to enter.

Cheers everyone, GONZO. <Rapidly running out of cash!!>