if the timing is off, then the exhaust valves could be opening up too early? But why would that just be on one cylinder. Just a guess obviously.
On one of the Tacos we owned when I had the timing belt done the shop said my old one was off by a tooth. I didn't notice a difference though, after they corrected that.
82 fj40, 99 uzj100, 06 gx470
97 DR650, 05 525EXC, 01 XT225, 89 TW200, 09 TTR125, 07 TTR90, 07 CRF50