Motorcycle Repair: 2004 YZ450F, aftermarket air filter, 2004 yz450f

Question
Hi, I have a 2004 YZ450F with some internal issues. I had a cylinder base gasket leak and removed everything according to the manual to repair it and installed all new gaskets. Once it was together the bike was extremely hard to kick over and would occasionally back-fire. I removed the "valve cover" and inspected the timing marks, they're aligned properly but it is still very hard to kick over. I have had the bike runnning by a roll start and it seems to run correctly but can't get it to kick how it used to and once you shut it off hot its nearly impossible. I'm at a loss. Please help. Thanks.

Answer
Zach, There is a timing trick on these called a cam mod. Is there an aftermarket pipe on this unit or an aftermarket air filter? If not use the stock timing also check the valve clearance between cam and shim bucket it sounds like you have lost most of the clearance. They will start hard and wont start hot because the valves are being held open. Even "Hot Cams" is very specific about their clearance on their new cams as it can damage them.