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?