Old Cub LT1042 with a Kohler Courage engine wont crank.
 

Hi, I have been working on a 2007 Cub Cadet Lt 1042. It crank's, but it just turns over until it backfires out of the muffler it will not start. I have checked everything I know to check. I replaced: Carb, fuel lines, fuel filter, changed oil, replaced spark plug. It has spark, it has compression, the valves are moving correctly. I had it running 3 days ago. Left to go get a wheel came back and it wont start again. If anyone has any more ideas where I can check I am wide open to ideas.

Not saying it's your problem, but you may have a sheared flywheel key.
When the key shears, the flywheel gets out of position on the crankshaft and the ignition timing is thrown off.

Another cause could be valves out of adjustment.

By the way, if the engine is turning over with the starter, it IS "cranking", it is just not starting.

I bought the flywheel key yesterday, I haven't replaced it yet. Gonna work on that tomorrow. The engine is turning over, it is cranking you are right. Thank you for your feed back, I changed the post.

it was the woodruff key. It was sheared in half. replaced and running great.