I'm mystified, and this freaking bike and its mystifying problems are REALLY getting on my nerves. After lots of niggly problems on my 02 640 Adventure, I finally solved two big problems... leaking water pump seal, and then a cracked head. While the head was (probably) cracked, I'd ordered in a bunch of jets from the UK, and got the bike jetted pretty well (I'm in Johannesburg, which is at 1750m/5500ft). But that crack in the head was not good. I s'pose. Finally sourced a new one, and rebuilt the top end last weekend. Bike fired right up, first press. No oil leaks anywhere, water level steady... but the carburettion had gone out. I assumed it was because of the coolant getting where it shouldn't, but I just can't get it right. It starts pretty well, idles fine. But as soon as you crack the throttle hard at low revs it just disappears -- no bang at all. Close the throttle again and it picks up right away. If you wind it up slowly you can get to full throttle - a bit down on power and hesitant. I started with 45 idle jet (I think, little bastard fell out when I was changing it and vanished). I tried a 38, and a 34. The 34 I guess is right, because with the others the fuel screw did nothing. I moved from the 72DR emulsion tube to a 68, no improvement. I moved from 140 main to 138 main jet, no improvement. This size is weird -- for a 640 with a PHM 40 even at altitude this seems very small. I then tried a 158, and now it won't start at all. Which is weird, since it starts on the starting jet (a 60) and maybe the idle jet, the main jet should not come into it at all. I even tried bump starting it several times downhill. So, masters of the thumper -- any advice? I've pulled the carb apart, checked float level, everything looks reasonable inside, needle&seat is sealing properly, vents are clear, etc.