'75 350 Backfires thru exhaust

[Dave Z]

After heavy accelleration, then backing off the gas completely, my 350 backfires once through the exhaust (not thru the carb). I gave it a complete tune up and set the timing. I know this has to be a simple problem, and it's been a number of years since I have worked on a 350, but it has me a bit miffed. The exhaust system is old and rusted through in very small spaces. Could this actually be my exhaust system taking in air at the manifolds? Any ideas would be greatly appreciated.

Thanks - Dave z

 

[81kix]

Coil?

 

[Vette/Berlina-coupe]

-Still other possibilities to ponder...

-Burned Exhaust-valve...(can ignite tailpipe gases via flame emitting from comb.chamber upon power-stroke)? Worn Timing-chain...? To rich of fuel-mixture...? -No, couldn't be the Ign.-coil ! See if it still backfires when you are coasting and you turn-off the ignition (unfortunately you may blow a hole in an old muffler when you turn the ignition back on), if so then it would be an after-glow in the exhaust-sys. is igniting unburned fuel-mixture, if not then it would appear to be associated with the valves, so you should run a compression check with all sparkplugs removed (or put her on a scope to pin-point the culprit).... ~Bob Vonheck
commentary: Tom.B suggests a very likely culprit to check for sure; --but if not that, then you can readily check the Timing-chain by pulling all the sparkplugs, then remove the Distrib.cap, and observe the Distrib.rotor movement while someone under the car turns the crankshaft(with a socket-wrench) both directions to see how much slack is in the Chain (remember the 2:1 gearing between crank and faster turning camshaft, but regardless there should be near no slack)!!! Please let us know here.... ~Bob vH

 

[Dave Z]

Thanks Bob. What you mention about turning off the car while coasting then starting again, I used to do that on purpose to startle people in my old '71 cutlass (I was 16, it was 1978...Daytona Beach):L .

Seriously though, at this point I was am still under the impression it's a timing issue, and the timing chain is foremost on my mind.

 

[curtis]

muffler and tailpipe explosions usually caused by late ignition timing or late valve timing or burnt exhaust valve. could also be a broken or weak exhaust valve spring. I would pull the valve covers and take a real good look up top before going to the trouble of changing the timing chain.

 

[Tom Bryant]

A bad AIR pump diverter valve will cause a backfire when the throttle is suddenly closed after acceleration. Allows fresh air to be blown into the exhaust manifold where hot unburned fuel is presant. When you quickly accelerate and decelerate the valve should divert the air charge away from the exhaust manifolds on decel only. If it is sticking then you get the backfire.

 

[Robin7TFour]

Dave, now i know who that annoying kid was! trying to enjoy the beach,and all this booming going on...

Robin

 

[grapeknutz]

Allso check the carb float level it could be too high and momentarily causing a over rich condition.

 

[Dave Z]

What a great webite...great suggestions and great laughs everybody, thank you for your imput...and Robin; apologies for being a very annoying kid during your beach vacation all those years ago...it's what us locals did make people's vacations a memorable one .

I beleive the diverter vavle is part of the PC system? The sad part is, I have yet to remove all the emissions garbage from the motor. It 's something I can live with until I get around to this though.

 

[Tom Bryant]

If your AIR pump has a seperate belt you can remove it and see if the problem goes away.

 

[JohnZ]

When the diverter valve fails (diaphragm ruptures), not only does it blow air into the exhaust manifolds all the time, but the ruptured diaphragm creates a major vacuum leak through the hose from the carb to the valve - plug the hose. Leaks in the exhaust system will also cause "after-firing". Forget about the timing chain - that's not causing it.

 

[Dave Z]

Problem solved...thanks everyone. I plugged the hose with a 3/4" x 2" Hex Cap Screw and tightened the clamp around the threads, then capped the smaller hose leading from the AIR pump to the carb with a small screw. No more backfiring through the exhaust.

 

[Tom Bryant]

Don't you just love an easy fix?

 

[Dave Z]

Easy fixes ROCK

 

[Robin7TFour]

okay dave now bring yourself down here ,so i can un-fix it . for all those booms at daytona ya did!!