Hey guys, I'm having a problem I just cannot figure out. Please help me get to the bottom of this. I've searched all over TT and have not found an answer. I have a 99WR400, YZ timed, all free mods, stock exhaust head/mid pipe with FMF PowerCore IV can. The bike has run very well and been very reliable. The stock carb started leaking from the overflow regularly due to a damaged needle valve/seat in the float bowl. Rather than replacing those parts, I came across a carb off a 01 WR426 for very cheap ($65 for carb & 01 WR426 head pipe to replace my crappy stock 99WR400 head pipe that blocks the oil filter cover). So...I pulled the 426 carb & my 400 carb apart. The 426 carb is the newer generation Keihin FCR flat slide with the integrated hot start & the internal accel pump. I had my old carb jetted correctly and dialed in w/ a ZipTY fuel screw, so I wanted to swap my old jets into the newer carb. Here are my initial jet settings: -main: 172 -Pilot: 45 -Pilot air jet: 65 (i hope i'm not confusing the Pilot & PAJ here) -needle on 4th clip down from top (was set like that when i got it) So I clean the carb, everything looks good, and I put it on the bike. I give it a couple kicks, and notice I had to plug the vacuum line on the head, since the new carb has internal hotstart. I do that. Kick it again, the bike won't start. I pull the hotstart on the carb to see what happens. 1 kick and it fires right away. As soon as I push the hotstart back in, the bike dies. So I figure its running way to rich. I adjust the fuel screw pretty much all the way in to compensate, kick it without the hotstart, still won't start. So I pull the hotstart out again, bike starts immediately. The fuel screw seems irrelevant. So now I figure the bike is set way to rich, since it will only run with the hotstart leaning it out. I pull the carb to check the needle, and it was on the 4th clip down from top. I move it up to the 2nd clip down from top to lean it out. Put the carb back on the bike, fuel screw at standard ~2 turns out. Bike still not start, not even pop without the hotstart on. So now I'm very confused. I pulled the hotstart plunger off the bike and clean it out. Put it back in, same thing. What the hell is going on!!!?? I figure my jetting should be pretty close, as the bike ran excellent with these jet settings on the old carb. Why will the bike only start with the hotstart on (plunger out), and dies as soon as I turn hotstart off (plunger in). Why does the fuel screw make no difference!? Please help!!!!! Evan If theres any pictures or more explanation I can provide to help let me know!