Renas, Not sure if you have tackled your problem yet, but just in case want to share my experience with the bike which had exactly the same symptoms as yours of not wanting to start unless you pull out the hot start knob. I have a 2000 WR400F which I have been riding for two seasons (from April till November). It is stock, no mods (apart from de-baffled muffler). It has never caused any problems to me as I serviced it regularly (usual stuff as per the manual). This month I disassembled part of the bike including the carb which apparently needed to be cleaned and checked. After putting the whole thing together the bike did not want to start either with choke on or off and only showed the signs of life if the hot start knob would be pulled out. This is in my garage where it is always hot due to the fact that the house heating equipment is there. Anyways to cut the long story short, I tried all and everything to figure out what was causing the problem with no success, though. Finally, (having messed with the carb for a couple of weekends) I looked at all jets after I took the carb apart again and noticed that both main air jet and starter jet were almost of same size and had identical thread. The difference between the two, though, was that main air jet, as you can easily tell, has a much bigger hole in it (for air flow), whereas the starter jet has a tiny one (for fuel flow). I was stupid enough to misplace these two buggers and that had resulted in very rich mixture (less air intake and excess fuel). I swapped them aroud and, bingo, it runs sweet and steady again after you kick it just once. I am not sure if you had disassembled your carb like I did, so maybe this is not your case. Hope you have your bike alive again. Cheers.