I am facing this RPM problem with my single-cylinder engine(Duke's) that I have used in my project.. Even though the ECU is programmed to allow RPM upto 12,000 revs but still engine can only reach 6,000 when the clutch is engaged...it can go near to 12,000 only while the clutch is disengaged. What can be the reason for this... Also , when I disengage the clutch, a sudden large amount of air is sucked in. P.S. - The engine is also having problem of unburnt smoky fuel coming out of the intake port, which I guess is due to the exhaust back-pressure as the exhaust pipe & muffler is custom-built & not the engine's standard one.