SX/SXi/SXi Pro 650sx back firing

[Bruins]

I'm working on a 650sx that when we try to start it, it backfires out the exhaust, stopping the driveshaft and shearing the flywheel key, we have adjusted the timing throughout the range and it does not make a major difference. Any advice?
Kiehen carb
145psi/cylinder
leak down tested ok

The flywheel the ski came in the shop with had "BTDC15" on it, my thoughts was this read Before-Top-Dead-Center-15deg. I tryed another flywheel and still the same.

the customer stated that they swamped it (Hydrolocked) and hasn't ran since, the motor was un-swamped after they did that.

any ideas??

 

[Matt_E]

I would check out the reeds, especially since this happened after "swamping".
People will try to blow out the water with the start button instead of draining it. This can break reeds. Broken reeds can lead to backfires. Maybe that's not it, but it's worth checking out.