I have a Scotts tractor with a 17.5 HP (13 kw) Kohler Command engine in it (Model CV490S). It was running fine with no problem, then was shut down. Since then, I have tried over numerous days to restart it without success. It will not start, but simply cranks without sounding like it wants to start. The only time it does sound like it has fire is when the choke is engaged, at which point it loudly backfires! I have replaced the spark plug and the fuel filter. I have checked for fuel flow, removed and inspected the carburetor, and removed the valve cover. Everything appears to be OK, but the engine will not start/run even roughly. I have tried using engine starting fluid, which also results in a backfire. Any suggestions? So far, the only things I can think of involve the fuel solenoid, the ACR (auto compression release), ignition module, or valve problems. Any other ideas, or any suggestion on what to try first?