I'm having a heck of a time with my '00. It's got a lean spot, bigtime, at 1/8-1/4 throttle, and it stays there, bucking and surging, slowly picking up average revs, but not improving, while I hold my wrist still. I think this rules out AP issues. It does have a heavy O-ring on the AP timing screw. I've replaced the fuel screw, but am willing to go back to stock. It's a JD screw. I have pulled and cleaned the carb (it was pretty darned clean already). AFAIK the vacuum plate is fine. The PAJ is fine (stock, 100). The MAJ is fine (stock, 200). The AP diaphragm is fine, unaltered. I installed a JD needle (his blue one), fifth clip, and a 168 MJ, which cleaned up the top end dramatically, and left me with this untractable problem down at part throttle. I have tried, in no particular order, these pilot jets: 40 PJ, 42 (OE) PJ, 45 PJ, 48 PJ. None of the richer PJ seem to help the problem. The 40 PJ can make it worse, depending on the position of the fuel screw. With the 48 PJ, I can make the idle worse with the fuel screw, to the point of stalling the bike, but otherwise it has no effect on the throttle as I crack it open. No matter what PJ is in it, it pops on decel. The bike is supposed to be my motard/errand runner and dual-sport (so I can ride it to the OHV area instead of getting my oversized trailer to go ride for a few hours in the dirt, which is a drag). It's going to be unrideable in traffic like this, and just about unrideable leaving corners on asphalt or dirt. If I rule out dirt in the fuel-screw (idle mixture/pilot screw) circuit, and rule out Pilot Air Jet issues, because of my cleaning and visual verification that carb cleaner shot in the right places resulted in a jet of carb cleaner leaving the right places, I think I have to have either: 1. A vacuum leak. Guessing this would have to be the cylinder-head boot/adapter. Possibly cracked. I think I can check this at idle by spraying carb cleaner at the junction between it and the cylinder head, but I honestly don't know if that's conclusive. If I have no joy with anything else, I'll order one and see if that's it. 2. Valve lash. This would be the valve sucking up into the head and the base-circle of the cam keeping the valve open, right? A lack of clearance? Before I pull the cam cover off, does this sound right? It's a noisy motor, for sure. Am I missing anything else? The motor had the YZ stator and flywheel replaced with an '01 WR flywheel and stator. Dunno if that's any heavier, or if it would expose any of the issues I've got. I don't think there's any adjustability in spark timing there, so I don't think that's an issue. Thoughts, before I pull the cam cover and write further testing off for the weekend?