Big Backfire !!

[KimVanOrder]

So: When I turn off my '28 I get a great big "KABOOM" Backfire about 10 secounds after all is stoped. I was actually walking away from the truck when it went off. Any thouhgts as to what is causing this and what to do to fix it?

[Reds34]

Fuel in in the exhaust?

Red

[Stakebed]

My riding lawnmower used to do this if i shut it off while the rpm's were too high. Heat in the muffler/exhaust will ignite the fuel thats being sucked in after its shut off.

How do you (step by step ) shut off the truck? do you push the spark advance up let it idle down then turn the key off?

[KimVanOrder]

When I shut it off I stop, let it idle, then turn the key off. I leave the spark control about 3/4 down. The engine is compleatly stopped, all is quiet for 10 to 15 sec. then it goes off some times. The other day I turned the GAV (choke knob) all the way shut about a mile from home and then when I shut it off all was OK.. So,,, I think it was running too rich. Loaded the muffler with gas and carbon then Bang... Can it be that I need to redo the GAV?

[Stakebed]

yea if your running rich it can eventually collect enough in the muffler then when you stop the fumes start to rise back up the pipe and hit the hot manifold.

im speaking from more modern carbed cars/fuel injected/lawnmower experience here but all the principles are identical

[ConversionT]

My '25 Shaw Conversion T will do the same thing if I don't let her "settle down" (and I run her pretty lean). I back the throttle off completely and retard the spark (all the way if she's warm and won't die on me) and let her idle like that for a minute or two...or three. No backfires if I do that - ever.