47 Allis B runs bad warm

My dad has my 47 B with a 5' woods mower on it. About 2 years ago I had a mechanic friend of mine rebuild the engine, new sleeves pistons etc. valve job, new used block, valve job, and I even had a new coil put in the mag. It has done a real good job for the past 2 years and I'm at a loss, what could be going wrong? I pulled the plugs and they all looked good, It has a miss and when you turn the mower on after it warms up it will hardly run at all if there's any kind of load on the engine. I messed with the carb screw and choke setting that didn't help.
My first guess would of been that the coil was bad, you know when they get hot they don't work as well... but like I said its only 2 years old and I am not very good with taking apart the fairbanks mag and putting it all back together correctly, my mechanic friend moved away and I'm kinda at a loss... any suggestions are welcome!
Thanks!
Rob
 
I would start by checking the spark as soon as the problem starts. You need to have a blue/white spark that will jump a 1/4 inch gap. If you find that the spark is weak it could be as simple as needing new points. If the spark is good it may need the valves adjust because after so many hours they close the gap so they need to be adjusted again to specs. As for the mag send me an e-mail and I'll send you the manual for your mag
Hobby farm
 
You didn't install a new set of Champion plugs, did you? Does it run better partly choked? That could mean either weak spark or choked fuel system. And yes, the mag could cause the problem, possibly as simple as a cracked cap.
 

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