I tried the coil that didn't solve it - Cars & Trucks

Look at this forum for tractors.
I hope it helps you!

http://www.tractorforum.com/f192/ih-674-78bhp-running-under-load-issues-17715/

Another thing that can cause this is the carb. It may be getting too much fuel once it's warmed up. An engine needs more fuel when it's cold than it does when it's warm. Cold engines run inefficiently so they need more raw fuel to fire-up, and stay running. This is all a choke does. A choke plate closes-off the air going into the carb which makes the fuel mixture really rich so it can run. Less air = more fuel. You'll also notice that cars with chokes also idle real high until they warm-up. This fast idle and the over rich mixture (due to lack of air), allows the engine to fire up and stay running. As the engine warms-up and starts burning fuel more efficiently, the choke plate begins to open, which slowly lets more air in and leans-out the air/fuel mixture. Once the engine is warmed-up, the choke plate will be wide open allowing the carb to run as it is supposed to with the correct metering of fuel and air.