as a far fetch thing if you don't find it, check and make sure the impulse spring has not snaped.... the timing of that mag will float by 25 deg then... not likely but a thought....
also a broken tooth in a mag would let it float/change/detonate
or plain mag internaly timed wrong..you did say new rebuild.......
and this also could be as simple as a cunk of something "glowing" in the cyl/plug causing it to fire early/detonate..(but i would think this would show up sooner)
but i like all the fuel starvation ideas expresed first....
just some thoughts to add... shoot down at will.....
flew for little less than an hour last night and it ran well. I ran it quite hard but air temp was cooler couldnt get more than 170 on oil temp. I.A. checked venturi and said looked good. says needs to be checked every 100 hrs. He also did a fuel flow test with tanks full and drained 10 gals per side worked well and no vacumn present in tanks. All vents seemed to work well and it flowed more fuel than when i did it.Will fly again soon.
Engine history has 225 since factory overhaul. Then had a prop strike at 214 hrs then new crank installed and all ads checked and done flown 20 hrs since.