One of the most common problems that causes backfiring is too much throttle when turning the switch off...remember the gas is flowing constantly and when you turn the switch off no spark is immediate..the excessive throttle allows gas to get into the exhaust and ignited in the hot muffler...so make sure you cut the throttle back to full idle and let it run for 30 sec or so then shut it down..this allows all gas to be used up..My guess is the explosion was internal also and your oil seal is broken or loose..try tightening up the bolts around the crankcase..if that doesn't do it you need to get the gasket replaced..Just shut it down slowly in steps..the backfires will cease...good luck..
My guess is air filter dirty and fuel is not burning properly which causes heavy carbon build up on cylinder head. So when you shut engine off the red hot carbon is igniting fuel in combustion chamber like an explosion. That is what blew out your crank shaft seal.
SOURCE: LTX 1000 craftmans rider 18 hp ohv kohler back fires/gas coming
your intake vavle is stuck in the open position
remove plug and muffler squirt oil down cylinder hole and on the vavle and crank over several times to free up
SOURCE: Craftsman 42in. rider with a
Try adjusting the Intake and Exhaust Valves if this engine is the OHV model. I am also concerned about fuel flow and trash in the carburetor. This machine needs a fuel filter . It also needs a fuel-shutoff valve so that if the needle valve in the carburetor sticks open or gets trash in it to hold it open, it does NOT let all the gas in the tank drain in and overflow into the engines crankcase. I would also drain the fuel, and check the float bowl for water/sediment and debris. If you find that stuff in there, clean it out, and the rest of the carb should be removed and cleaned also.
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