I recently bought an 07 YZ450f and it is having problems. I was hoping someone could help. This is a bit of a novel, but I’d rather give too much info than too little. The previous owner only had it a few years and didn’t know how many hours were on it, but I have a pretty good idea it had a lot. The clutch and timing covers were pretty well worn, it looked pretty used overall and it wasn’t running well. It was hard to start, dying at idle, and backfiring. Owner thought it was a carb issue. It was dusty from not being used and the previous owner said it had jumped timing a few times, but he had recently replaced the cam chain and that hadn’t happened again in the few rides he had taken since. Maybe I should have walked away, but it was a pretty good deal so I decided to get it. As far as things that could be having an effect on this problem, the bike has an aftermarket FMF Ti Megabomb header, Powercore 4 exhaust, and an aftermarket Boyesen Quick Shot 2 accelerator pump. The jetting is as follows: Main: 175 Pilot: 48 Leak: 45 (new, the 40 that was in it seized in the float bowl threads) Starter: 72 Needle: NFLR-4 Fuel Screw: 2 turns out I am in Northern California not much above sea level and it has been 80-85 degrees when I have been messing with everything. When I got it home I completely tore down and cleaned the carb. Where fuel sits in the float bowl, on the float, and the jets had a green/yellow coating that almost looked like pollen. It was kind of powdery but I couldn’t just wipe it off. I cleaned everything out with Simple Green, then hosed everything down with carb cleaner and blew it all out with compressed air. I also cleaned the airbox, the air filter (wasn’t terribly dirty), and oiled the filter with fresh Maxima FFT. The spark plug that had been in it was the wrong type so I replaced that with the correct one, a new NGK CR8EIX Iridium plug as well. I drained the gas tank and put new gas in. I removed the existing worn out and cracked exhaust gasket, cleaned all surfaces, put in a new gasket, and used high temp copper silicone to reassemble the header to the head. Once I got everything reassembled I got it to start, but only with the choke on and the idle adjuster turned all the way in. Once it starts I can turn the choke off and it continues to run. If I try to lower the idle (I would guess it is at 3k-3,500 RPMs, but no way to measure) it dies. If I try to give it gas it backfires when I turn the throttle and wants to die. I thought it may be a stuck hot-start plunger, but with it running and the choke on using the hot-start lever increases the RPMs. I messed around with the fuel screw, adjusting from 1.5-2.5 turns out, but that didn’t seem to make a difference. As I see it there are a few things I can do next. Maybe the jets still have some of that green/yellow coating in the passages making the bike run lean so I may just replace them all with new jets. Alternatively the cam timing may off, either from the work the previous owner did or it jumped timing again. As long as I’m in there it makes sense to check valve clearances and take a peek at the cylinder/piston based on what the previous owner told me anyway. Then there is Plan C, but I don’t know what the heck that is yet! Before I go too far down the wrong road I thought I would see if anyone has any suggestions. Have you seen or heard of anything like this before? Is there anything I should check or do first before replacing the jets and checking the timing? Thanks for the help!