I have been working on the gas motor on the JD 3300 combine for about a week now and it is still acting up. When it is cold it runs fine, but as soon as it starts to heat up it starts sputtering and backfiring. I do not have much experience trouble shooting gas motors and hopefully someone here will have an idea what the problem might be.

Here is a list of things we have done since the problem started (some are not related but while the machine was down we did them anyway). Rebuilt the carb, alternator and water pump. Replaced the fuel pump, blew out the fuel lines, cleaned the sediment bowl and filter and cleaned and checked the air filter and pre-cleaner bowl.

 I don't know a lot but I am wondering if it might be a head gasket? I have never had a blown head gasket before to know what to look for. What do you guys think? Thanks