Hello, I am new to the forum and sport quads, so please excuse me if I have a few things mixed up. I have a 2005 YFZ 450 that I need some Jetting info on. it has a fair amount of work done to it, mostly by the previous owner. It has a Yoshimura pipe, pro design intake with K&N filter and no air box lid. It also has a fully rebuilt engine, with a decked head. I don't Believe there are any bigger cams in it, but I could be wrong. Anyway, I am in the process of breaking it in from the rebuild currently, and noticed that it seems to run on the lean side. it backfires a ton, and then shuts off some of the time. I figured I would stop and diagnose the problem before finishing the break in of course. I pulled the jets out to see if they were too small and they were actually pretty big. it has a 175 main and a 50 pilot. (according to RockyMountainATV/MC.com the original jets are a 42 pilot and 158 main) I then pulled the plug, and it looked pretty white. I had the air/fuel screw at 2 1/2 out and brought it to 3 1/4 with no change. I am now stumped because those seem to be some pretty big jets, and it is still running lean? It now won't start after trying to warm it up. I don't want to bring it into the shop AGAIN if I can fix it myself. all the information I have on it for a previous setup is that it had a "Ron Wood" intake with a UNI filter that was rubbing the air box, and seemed to have got a little hot judging by the flat spot on the filter. So I replaced it with a OE breather tube going to the air box, and a K&N filter with a pro design adapter. I was told to spray some carb cleaner/brake clean on the intake boot and exhaust to see if it would rev up. It did not. I have not checked the needle position, but I figured it would start and run. any help is appreciated, and I have included a few pictures if it helps. Thank you!