VAC Backfire

[Paul f]

My VAC has developed an occasional bad habit. After she is shut down there is a 1-2 second pause and then ka-pow. Just one. Startled me the first couple of times. This is a new bad habit. No changes have been made to ignition timing, carb settings or the exhaust system. She starts and runs great just this backfire...

 

[rlp]

I have a generator that does that too. ha ha
I don't really have an answer other than to check and see if your timing is set right.

 

[Joe (Wa)]

TRUE
Too rich or too lean will dump unburned fuel into the exhaust manifold because the burn rate is retarded.



OLD THEORY (maybe, maybe not?)
If the backfire is actually afterfire (up the stack) you're probably running too lean or way rich.

Too lean, exhaust is super hot, killing the spark dumps some gas in the manifold and it cooks off.

Way rich, gas is in manifold, vaporing but not burning for lack of oxygen. Shutting off allows exhaust to be diluted with oxygen and when to combustible mixture, gas cooks off.

Joe

 

[Jerry W]

My first guess would be what Joe said but I have also seen where a failing ignition condensor will also cause this problem although usally they will misfire and pop while running.

Jerry

 

[Paul f]

Thank you all.

Since I never see any black smoke I will assume I am running to lean. Will richen it a bit and see what happens. Thanks