Yamaha 30ae 689 backfires and wont start

[Check Mate]

Has 90 lbs compression on each cyl. Smokes like it is burning fuel then backfires. both plugs fire and fuel makes it to each cylinder. How do I check the timing?
Thanks
Check Mate

 

[JB]

Re: Yamaha 30ae 689 backfires and wont start

First confirm that all plug wires are correctly connected. Then check for a sheared flywheel key.

Good luck.