My son was riding this past weekend. The bike started bogging at just past 1/4 throttle. I checked the timing and it was off. I removed the the head cover and readjusted the the timing. The bike started and ran fine. He rode the bike for two laps and it once again started bogging at 1/4 throttle. Checked timing and once again out of time. What can cause I bike to jump out of time. Worn cam shaft sproket or possibly broken cam chain tensioner. I have just recently replaced valves,springs new piston and cyclinder and crankshaft assembly. I had checked valve clearance and they were good. Thanks for any suggestions.