I had the same problem with my 400-E. I sprayed an entire can of Holley carburator cleaner into air intake with the engine running of course & the air filter box removed. You'll have to keep the engine above idle speed to keep if from choking on the cleaner. If your not experienced with type of precedure be "carefull" . Don't let the fan blow it in your eyes. Wear saftey glasses. The problem is carbon build up in the intake. Some of these cleaners work much better than others so you may have to experiment to see what works best according to whats availible in your area.