Cyril,

I have no experience in setting up for a tractor pull. My guess is that you are setting compression for the load of a flywheel alone and when you put a load on it (via clutch?) the setting will be off. If that is the case remember when you adjust compression you are setting ignition timing. If you greatly change the load, you will need another setting.
You might try lowering compression a bit and see if it runs cooler but I doubt that you will get a compression setting that is optimum for both flywheel and max load. My guess is that you would set it for load and try to just keep it lit while on flywheel alone.

George