JD 318 backfiring


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JD 318 backfiring

Henry I've recently had a rebuild on an Onan-powered 318, which ran fine for a few hours, but now begins backfiring big time after it warms up. Rebuild was mechanical with new engine guts and deck clutch, but no electric or fuel parts. Engine cranks and runs fine for about 5-10 minutes before the backfiring starts. It's an old 318 that belonged to my deceased father-in-law, so I don't know much about it's history.
bontai Joe Have you retorqued the head bolts after running it a while? It might be as simple as sealing a slight leak from the head gasket from loose bolts.
Dave Henry - Do you know if the engine is an Onan B43G or a P218G? If it's a B43G, it would have points and condensor. If that's the case, I would check the points to be certain they are properly gapped. The wide open gap setting is 0.016". Also, a new condensor might be in order.

If that's not it, or it's a P218G Onan (electronic ignition), it may be the idle mixture screw on the carb is out of adjustment.

I sure hope this is not it: Sheared key in the flywheel. That requires an engine pull. However, I would think the backfiring would happen even before the engine warmed it this was the case.

Could also be the valves are out of adjustment or they are sticking.

This is going to be a process of elimination. - Dave

Dave Good point!!!
Alan Anderson Henry it could be three things.
Sounds like the exhaust valves were set too tight with not enough clearance. I believe the gap is 0.013 and I think it's 0.005 for the intake valves.When the engine warms up, the valves expand and with not enough clearance(gap)they will NOT close all the way during compression stroke.
If it's not this then your timing maybe moving around.
The last one is it maybe a weak condenser which could cause this. (That is if you have a 1983 to 1986 with points.)

Henry Thanks, guys, I'll give them all a try & see what happens. Anyone know what the head bolts should be torqued to?
Barnrazor Onan BG43 engine recently started backfiring and had no power. I replaced the coil No help..When changing the points I noticed the point gap to be extreme. I can adjust the point gap to 16ths. but on the next revolution it looks like a 1/4 in gap. Any Help
Sasha I have an 86 model with points. - it was backfiring after you ran it a while and then cut if off, and was really getting sluggish. i put new points in it and adjusted the gap to the specs in the manual. and that fixed both problems. - I now just purchased a 89 model and i think it has the different onan 18hp with electronics. - it is really slow and sluggish but doesnt backfire


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