Had to replace output shaft on a 2007 900 custom. So completely disassembled and reassembled motor. Replaces all gaskets and o-rings, also checked valve clearance and adjusted. Went to run it and on the rear cylinder it is making a pop sound when that cylinder fires and you can see the flame through the throttle (it flashes not a full flame coming out) as it runs at low RPM at high RPM it goes away. Also if you ride it, it does not have the power it should have at any speed. I double checked both timing marks (stator and gear on top). Used timing light on both cylinders and they are dead on. Checked cylinder pressure front is at 158psi rear is at 153psi (book says 142-228) so i have pressure i should this makes me thing it is not a valve issue (pressure would be way lower on rear) and makes me think it is a timing issue. But how? What am i missing or not looking at?