After checking the valve clearances on the two bikes (all sweet!), I decided to have a play around with the ACV circuit on my 450. First of all I put in a new spark plug, set the fuel screw and went for a strap for a baseline. The ACV is the cover on the left side of the carby, held on by 2 screws. I turned the diaphram around, and blocked off the port where the rubber o-ring goes with a tiny bit of plastic. I adjusted my pilot circuit jetting, I ended up going from a 52 to a 50 in the end, I tried 45, 48 then 50. Fuel screw set at about 2. I didn't have to change the needle position or main jet. I dont have any pilot air jets, I really need to get some. I feel I could get the pilot jet back down to about a 45 with the correct PAJ. I have found that the first noticable thing is that the bike pops a fair bit more on decel, not because of lean jetting (pilot circuit set correctly), but because of the run on. I believe this is from the really heavy flywheel driving momentum and keeping rpm's up, even with the throttle closed. The popping stopped when I tried dragging the rear brake a bit, simulating lighter flywheel of a YZ (which gives more engine braking). The run on did decrease from when ACV was intact, less fuel available for the engine to suck in. The thing that I really noticed is that the throttle response was a lot better. The bike seemed to rip a bit more through out the rev range. My jetting was right on before, but the less air and fuel from the pilot circuit really seemed to help. It is hard to explain, but throttle response was remarkably the same, every twist of the throttle. It seemed to be cleaner. That may puzzle you, but the ACV circuit is not constant, the ammount of air being sucked in and when it shuts off the air always varies. This is what screws around WR's jetting, and every jetting circuit is effected. My bike idled very sweet, remarkably cleaner. I found I could turn revs down a lot more than I could before. This could be to do with smaller pilot jet being used. I need to get some pilot air jets (hmmm, 52 pilot to start with, PAJ must be too big!) and have a play around a bit more with this, I think if you could put up with the increaced popping, there is gains to be made. Yes, the ACV works, it does it's job of stopping "lean condition" when you get off the throttle. It is a pitty that it is not constant, because the bike seemed to run better with out it. I definately think with a quieter pipe, it wouldn't pop so much, my procircuit seems to have great scavinging effect, which doesn't really help the run on. I must try disabling the ACV on my missus' WR250, her bike isn't so loud. It was fun to try, maybe you guy's with quieter pipes might have a bit more benefit. All I can really say is BRING ON THE FUEL INJECTION!!!