this is probably stupid but.... i had the same problem with my 85 200M and it was because i forgot to turn the gas cap vent to on. i was soooo pissed because i pulled on the dam thing for 20 min. then i saw it was on off. turned it on and it fired right up and ran like new.

 

yeah, my cap like doesnt vent when its switched on or off, it only vents when the dial is inbetween