That would be when the engine is developing the highest compression loads.
I know that 280000,310000, and 330000 OHV versions all have head gasket issues of failure between the cylinder and the push rod galley. I have seen these to be anywhere from no smoke to heavy smokers. The only thing they primary have in common is excessive oil usage and signs of oil in the carburetor air intake.
I even had one that would blow blue-black smoke under loads to only clear up in off load conditions. It had just enough head warp-age to allow a pressure leak under load. A new head gasket resolve the issue after increasing the head bolt torque to the new spec.
