My 2005 yz450 is super hard to start. It has a 480 bore kit. When I bought it, I changed the spark plug, air filter, drained the gas, cleaned the carb. And nothing helps. There is times when I can't even get to start. Incredibly hardd to kick, usually have to push start it. Once I get it started, it runs great, until I get to the top of a gear. At the higher end of all 4 gears, the bike will sputter, and pop. I live in Utah, my elevation is around 5000 feet. Changed the jets to bigger sizes to see. Currently have a 195 and a 48 in it. Thanks in advance!!