Backfire after parking

FortRockRider

FortRockRider

Junior Member
My 1930 A will let out a very loud backfire sometimes after I turn it off. It happed on the way back and forth to a local parade on a very hot 4th of July. It will also backfire when I drive up a long hill and park at the top. Am I running too rich or too lean? Or is this a carburetor issue? How to I start to diagnose this?
Thanks
 
I am assuming it backfires out the exhaust pipe and not the carburetor.

The exhaust manifold and exhaust system are hot and raw fuel is getting into it and exploding. After the ignition is shut off the engine will keep turning over enough to pump air/fuel mixture into the exhaust system.

Another possibility is that the ignition is somehow happening with an open valve which ignites any unburned fuel in the exhaust system. But this is very unlikely.

There are several things you can do. You can idle the car for a while to cool down the exhaust system. Or you can turn off the gasoline and wait until the engine stops because it ran out of fuel, and then turn off the ignition.

Make sure the timing is correct and the air/fuel ratio is correct. There have been many things written about this here on the Barn. Use the search function. Search for "timing" or "GAV adjustment" or similar.

Welcome to the Barn.
 
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Check for exhaust leaks. Sometimes air is drawn in to make the unburned fuel in your exhaust to ignite.
 
katy said:
Try: retard the spark before turning the engine off.
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Yes! And also make sure the hand throttle is not down at all, it must be normal slow idle.

Your throttle may not be returning fully to idle. Look for carpet or rubber mat or something preventing the gas pedal from coming all the way back up. Oil the throttle mechanism (behind the engine block, under the hood) to make sure it snaps back fully to idle. Check that mechanism for a broken return spring that needs to be replaced.

In general, backfire out the tail pipe (sounds like a gun shot) is unburned fuel in the exhaust system getting ignited. Too much fuel, not enough air, possibly spark ignition missing here and there.
 
Thanks for the comments. The GAV was open 3/4 turn so I've closed it to 1/4 turn. I'll see if I can oil the throttle mechanism and will try shutting down with the gas valve closed. I'll let you know if it works. Rain today so no A.
 
I had similar problems after fast runs . Now after a fast run I let it idle for about 6 seconds before switching off the iginition. Sorted!
 
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