Mower Engine making exploding sounds

Jerry,

You need to replace the missing muffler and do a tune up. Clean replace the spark plug and gap it. Go on-line and be sure the right plug is being used. While on-line get the valve clearance specs. Adjust the valves. Replace the oil. Put in fresh gas. Once you've done all of that you can diagnose any problems that occur.

Dave M.

Sounds like an ignition failure

Unburned fuel is exploding in the hot pipe...should you be messing with this considering your views on chainsaws?

Back firing? Result of too rich mixture. Carb. and/or timing issue?

I can try to lean the mixture via the carb screws. As far as timing, I dont recall ever seeing any sort of timing adjustment on small 4 cycle engines like this. This is a 12.5 HP so it may have more to it than the common 3 or 3.5 HP mower engines. I've worked on plenty of those 3 to 5 HP engines, and the only "timing" adjustment was to make sure the coil was the thickness of a sheet of newspaper from the flywheel, and of course point gap. Those must be correct, but that's not a "timing adjustment".

And from tearing apart many of those 3 to 5 HP engines, I dont recall any way to adjust the valves either (aside from replacement of the valves or the cam).

Timing would refer to a partially sheared fly key...and if you re-seat the valves (lapping) you have to re-gap...

I have heard that cool air is sucked in by not having the muffler on and can warp the exhaust valve or cause other problems by having cool air sucked in.