I bought a used ZRT600 with an extended track that had sat for a couple of years and the mechanic went through the motor and replaced what needed to be. It ran great last season and I put a few hundred miles on it. At the beginning of what I had hoped would be another snow filled year I could not get it started. I got it into my shop pulled off the carbs, cleaned them, replaced plugs, replaced fuel lines and oil lines, primed the oil. It now starts fairly easily and runs great when it is cold. If you take it 1/4 of a mile it overheats and backfires and will not run without the choke on. I have checked compression and I have 110 per cyl on all three. I have replaced the thermostat and cap. I checked the old thermo and it worked fine. Does anyone have any ideas on whats going on here and how to check it out further? I would like to at least have this working right before I put it away for the summer again........... HELP!!!!