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Have an old B&S riding mower that broke the connecting rod a year ago, and I cannot get the thing running right. Model 31H777-0202e1. Since I couldn't find a replacement mower at the time I decided to play with it myself. I have no experience in this, but that's why there is YouTube. Replaced the rod, piston, rings, carburetor, linkage springs, gaskets, etc. I lapped the valves and have compression around 120. Replaced the armature coil and have consistent spark. The flywheel key is intact. The timing marks on the camshaft are aligned. Valves are adjusted to spec. However, when I start it, It runs, but is very loud, like a continuous backfire. Fuel vapor comes from the air intake, and if you hold your hand over it, you feel pulses of air. The new spark plug has black soot indicating too rich. Adjusting the little screw on the new carburetor seems useless. So, what am I missing here, or should I just get a new mower?

Sounds like the head gasket has failed.

I replaced that recently after carefully cleaning the old one off and surfaced it with 220 sandpaper. Torqued the bolts to spec. I don't see any white smoke indicating oil burning, just a white fuel vapor that smells like the gas coming from the air intake on the carb. I did notice today that the intake valve moves a much greater distance than the exhaust valve. Is this normal?

check the oil for gasoline in it. Maybe the float needle is stuck. If it has a vacuum operated fuel pump make sure that is hooked up right.

DMzeus said:

I replaced that recently after carefully cleaning the old one off and surfaced it with 220 sandpaper. Torqued the bolts to spec. I don't see any white smoke indicating oil burning, just a white fuel vapor that smells like the gas coming from the air intake on the carb. I did notice today that the intake valve moves a much greater distance than the exhaust valve. Is this normal?

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No.
my guess is either a cam lobe has worn or the exhaust guide has moved in the head.

Scrubcadet10 has got it! I saw a video of a generator that was doing the same exact thing, and it turned out to be the exhaust cam was worn. Pulled mine today and sure enough. Just a nub. This leads to the obvious question of do I need to do anything to prevent that from happening? I suspect valve clearances that were ignored for years may play a role. The engine is from 2006, so time takes a toll. Thanks all!

Well looky here, Ol Scrubcadet finally got something right.
If i recall, those inteks occasionally had the issue of the lobe wearing... but it was more common on the V-Twin intek engines.
I would just advise run a good air cooled engine oil such as Kinetix, Kawasaki K-Tech, or Briggs & Stratton Branded oil.