So I am new to this forum but I recently purchased an 04 yz450f and I just took it out to the Mojave desert in CA this past weekend to give it a shake down. The bike rode and performed perfectly when it would start. My buddies were on 426's and they could kick their bikes over so much faster that I could kick the 450. It seemed that If I got a good first kick the engine would turn over and run but if I didn't get it on the first try it would take 10-15 kicks before it would finally turn over. It didnt seem to matter whether the bike was hot or cold, I used the hot start, I wouldnt use the hot start, I tried to figure out what would make it kick over the first time but I couldn't figure it out. Now the guy I bought the bike from said that it was a practice bike for a racer back in 05 and then the engine was rebuilt for racing in 2010 and then sat until I bought it. The bike does feel like it has had some work done on it but I dont know for sure without opening it up. If the engine was built for race fuel (not sure if it is or not), would me running premium 91 through it cause it to have such trouble starting? Sorry for the long post and for being new to the site. Any insight you guys may have would be much appreciated.