Hello, I just bought a 2002 YZ426F, and I'm having a little jetting trouble. I usually ride MX at 5000ft. elevation, I put on an FMF PCIV spark arrestor, and I was wondering if I should go leaner on the main jet. I currently have a #158 main and it backfires and pops alot. I've also fouled two plugs in three rides! Believe me I'm full throttle alot on the track. I have had two tanks of gas total through the bike. I have not re-checked the valves yet. I run 93 octane pump gas which may or may not contain ethenol, I don't recall looking at the pump. Can anyone offer advice? Thanks.