I have a John Deere RX 75 riding mower. It is extreemly hard to start. I have replaced the coil, spark plug, and even the carb. I did manage to get it running the other day and ran it or about three hours. I shut it off to see how it would start and it would not. It cranked but would not start. It seems to backfire out the carb. I pulled the flywheel to see it the woodfrff key was sheered and it was not. What else could make change the timing?

Hi,
You Deere RX75 uses a Kawasaki FC290V engine. It utilizes a two piece ignition in most cases that has a coil(in the usual place above the cylinder and adjacent to the flywheel.) It also has an igniter(a small silver rectangular piece) that attaches to the engine externally near the dipstick and has one or two white wires running to it. Most probably the igniter is the problem, though the igniter has been known to damage the coil and vice versa. You describe a "classic" igniter woe. If it can be done(depends on the flywheel) the best way out is the unitized ignition. Available only as a Kawasaki part under the #21121-2084.
Because Deere did not offer both ignitions on the model engine, I have no way to look it up. You will need to find the engine ID decal on the side of the block and copy down all the numbers, especially the series designation. It will be in the form of FC290V - BS02 or CS02, etc. The part with the "S" in it is the series. Kawasaki will not look up parts for you, but they can possibly find out if your flywheel is compatible at:
KAWASAKI MOTORS CORP. U.S.A. ENGINE DIVISION
5080 36TH ST. SE
GRAND RAPIDS, MI 49512
PHONE: (616) 949-6500

Sorry for the length of the explanation. The igniter is part #M73484 from Deere.
Peace,
Clarke