While the engine is at idle RPM, spray a short, light burst of carb cleaner on the gasket seam of the intake manifold where it fastens to the head. A change of RPM that coincides with your spraying indicates a vacuum leak at that spot. Any vacuum leakage there will alter the air/fuel ratio to the cylinder. Due to the nature of "harmonic vibration", a poorly secured gasket location might in fact "open wider" at specific RPM points, and will even erode the gasket if not repaired. The appearance that the crank thread repair is slightly "off center" may only be an illusion. It WILL NOT matter if the threaded hole is merely a thread or so off center anyhow...............the mounting bolt ONLY serves to secure the clutch and pulleys to the crankshaft.......it plays no role at all in centering the pulleys to the crankshaft. Centering of the pulleys to the crank is achieved by the bore of the pulleys fitting the outside diameter of the crankshaft PTO surface.

"will a occasional backfire cause any engine problems?" There are two types of backfire. One is an explosion in the engine's exhaust system, this is most correctly referred to as afterfire. The other is an explosion in the engine's intake manifold. When you lift your butt off the seat, the spark plug shuts down and because the engine is still spinning, unburnt fuel is blown into the hot muffler...KABOOM! Afterfire rattles your muffler a bit but isn't particularly harmful. I think the OP has true backfire caused by overly advanced timing (I'm guessing his flywheel moved relative to the crankshaft). That would explain why his tractor runs okay in the fast throttle position with a rich carb setting. -Deerslayer

I disassembled & re-assembled a Briggs and Stratton Intek OHV 18 1/2 HP Engine Model # 31P777, Type 0367E1, Code 050405ZA I made sure when I re-installed the Camshaft that the dot lined up with the timing gear dot. The Piston & connecting rod where not touched Made sure that the push rods are in their correct locations. After the engine was re-assembled I observed: Turning flywheel CW by hand where magnet lines up with Coil Magneto, both valves are closed, which makes sense. Turning flywheel CW by hand one full rotation where magnet lines up again with Coil Magneto, intake valve is open & exhaust valve is closed, which does no look right. With intake valve open & exhaust valve closed the spark will now cause the engine to backfire. How is it possible for that to happen?

Actually I started out looking at the valve positions by turning both engines CW through the strokes, but that made me dizzy, so I started to compare the flywheel magnet positions. That is when discovered that magnet position of the engine I am trying to repair is 180 Deg off from the engine with the cracked engine block Engine to be repaired 31P777-0367E1 flywheel magnet position with piston at TDC at 2:30 o'clock position Engine to be repaired 31P777-0367E1 flywheel magnet position with piston at bottom at 7:00 o'clock position Engine comparing to 31P777-0237 G6 flywheel magnet position with piston at TDC at 6:30 o'clock position Engine comparing to 31P777-0237 G6 flywheel magnet position with piston at bottom at 1:30 o'clock position Valve cover being in the 6 o'clock position Coil is in the 5:30 o'clock position The 31P777-0237 G6 has a cracked engine block due to low oil. I am now so paranoid that If I ever get the 31P777-0367E1 running, that I will check the oil level every time I am using the riding mower. I interchanged spark plugs from both engines, checked the spark gap, no change. I do regret that I did not engrave the position of the Crank shaft timing gear & Camshaft gear on both gears to rule out anything crazy going on.