Engine off backfire

[CW Spook]

Once in a while my 12-v M38 will backfire or at least POP about 5-10 seconds after I shut it down, usually just as I step out of the Jeep. I get the same type of backfire occasionally when letting off the throttle to downshift. It's very infrequent, so I haven't considered it a big deal, but I'm curious to know the cause. Any ideas?

[bmorgil]

The strange thing about a "pop" in the exhaust, is that it is caused when there is enough oxygen left in the exhaust to ignite any un-burned fuel. In other words it is actually lean, too much oxygen. When you shut it down, if things are good and hot, fuel is evaporating and being drawn into the pipe. If there is a leak anywhere in the exhaust system, or anything is hot enough it will light the vapor.

Its amazing how hot the exhaust manifold and heat riser can get. It will get very hot if the engine is running lean. I would look at a few things. Be sure the idle speed isn't too high. Check the idle mixture and see it is correctly set. If it is too lean it wont help. Check the ignition timing. Keep us posted on your progress!

[CW Spook]

Thanks! I've been suspecting that the idle was set too high. I think the timing is OK as it otherwise runs quite well. We'll start with leaning out the idle mixture.

[bmorgil]

Probably richer on the idle mixture CW! Set the idle speed with the idle speed stop screw and then the mixture. Tune the idle mixture on a warm engine, by turning the mixture screw in slowly until the idle speed begins to drop, turn the screw back out until the idle reaches a maximum and stop. Go back and forth until you are confident you hit the spot where the idle rpm's just hit the highest, and stop right there. Some like to turn it back in a touch right there. Then reset the idle speed one more time with the idle stop screw.