I have been working on a 48 A and to make it start it has to be idled all the way back and starts really hard. The tractor will miss when on a load. The carb has been rebuilt, and i have tried a different carb off a good running tractor. Tried a couple different mags. New autolite plugs. Valves are set rite. It will idle and run perfect once running but not under a load. Timing is good. I am not sure what else to try, i had the crankcase cover off to clean the sludge in the bottom and noticed it has aluminum pistons. It is almost impossible to turn over by hand, do the high compression pistons need a lot hotter spark to make it run?