died suddenly with an impressive loud backfire. never fired again. would backfire only once when trying to start, then not every time. found that evidently backfire split carb, had a 2x1 inch piece broken off top backside and steel top cap was split. new carb in, now has spark but no fire. only occasional backfire. timing search lead to broken bottom sprocket on cam/timing chain. replaced and set timing, adjusted valves, still strong spark no fire occasional 1 backfire. no fire even with gas straight into cylinder. going to check compression next. any ideas?