At what revs is it doing what? e.g. : How far is it reving to, and then what happens? Is this consistent? Will it rev through or does it stutter or just die straight away? (If it revs above 3000rpm, ish, I'd think it wasn't the ATU but fuel/air or overall bad running.) Will it tick over ok? Is it running well at tick-over and low revs? What happens if you can leave it ticking over for a while? Is it doing it just when hot or cold? Does it change when warmer? Other thoughts: Is there a good spark on all 4 and are all 4 zorst down pipes getting hot equally? Is the fuel filter/gauze in the tank clean at the tap and does it have an inline fuel fillter which could be blocked/dirty? What colour are the plugs and are they dry or wet? (are they in good nick and correct gaps?) Is the vent hole in the tank cap clear? (Is it a standard cap?) Is the air filter clean? Are the HT leads good and have sound connections (check for earthing out, sparking/smoking with Duck Oil, etc)? - and so on. Could be a lot of things. Check the obvious and then if no joy try to describe exactly wht the symptoms are and when, etc. Also, when exactly did they start and what had you just done (or not)... How was it running before this problem started and did it just start or has it got worse over time? Good luck ;-)