4 stroke backfiring.

LongboardMalinois

LongboardMalinois

Member
I picked up a Craftsman 4.5 hp 4 stroke lawnmower engine off the street yesterday, to hopefully put in a bike at some point. The problem is every time i pull the starter rope it backfires through the carburetor and spins the engine backwards, and it has already broken two starter ropes with the backwards spinning. The only thing i can think of that could cause this would be over advanced spark timing, but that seems unlikely since the magnet that triggers the spark is on the flywheel, which is very well attached to the engine shaft. I suppose the flywheel could have slipped a little on the shaft. Anyway, what do you guys think is causing this?
 
Pull the flywheel off and more likely the key is sheared or about to, It doesn't take much to knock it out of time..................Cuty
 
OK, I'll take the flywheel off and check on the key. I assume that to remove the flywheel i just need to loosen and remove the nut on top and then pull the flywheel off the shaft, right?
 
Right but may need a puller,you can make one out of flat stock. need a bolt in the center to push on the crank and two bolts out further to fit in the two holes that are already threaded to pull it...............Curt
 
Might get lucky tapping side of flywheel with mallet as you hold upward pressure.
The later aluminum flywheel models seem to pop off much easier than the old cast iron flywheels did.
 
Yep tap on the opposite side of the keyway,with pry bar under the flywheel. Croud but works IF YOU ARE CARFULL.................Curt
 
KCvale said:
Check your valves.
Click to expand...

Yeah....there should be at least two, if not there's a problem and it's not the backfiring!

KC should be a little more specific in what he means by "check your valves".

Backfiring can be caused by something as simple as a crack in the exhaust manifold or header pipe. Even just a loose header pipe can cause it.
 
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