My YZ450F 2007 is difficult to kick start and it only gets worst when the engine is hot. On top of that, the engine gets extremely stubborn, needing well over 15 strokes to get the engine running (with or without using the hot start lever). When the engine is running, it works fine. The piston, rings and cylindre were recently changed and the bike has been driven about 4-5 hours since then. Grateful for any good advice. Thanks.