i also have a 98 yz400f that wont idel but mine also has no bottom end power and if u try to take off it will bog
down and die and over heats. i am stilll workin on it to this day and its rilly starting to make me want to push it off a cliff. i have replaced every electrical component on the bike except for the tps
but it have tested it and it tests good. i have cleaned and readjusted my carb three times. i call around and found out that if the bike dosent have at least 130psi of compression it wont run rite so i ran a compression test on in and it had 65 so i put a high comp rebuild kit in it and it brong it up to 95 so i took it to a shop and they ran a leak down test on it and said they think its leakin out the exaust so i pulled it apart and reseated all my valves and adjusted the valve clearance and that still didnt help. so the last thing i tryed was puring a stall amount of oil in the cylinder and ran another comp test on it and i got 150psi and i noticed that every time i pull the valve cover off there is no oil. so im thinkin i might have a problem with my oil pump or a cloged line or filter. but id say it wouldnt hurt to run a compression test on your bike and if anyone has any info that would help me i would greatly Appreciate it.