[Gerry Zagorski]

It usually means lack of fuel when it only happens at the higher RPMS.. Check your fuel lines as Dan mentioned and it's not always visually obvious if they are collapsed. You might also check to be sure the fuel tank vent is not clogged and the anti siphon valve. if you have one is clear. If you have a primer ball that could be it as well..

If you want to see if any of the above could be causing this what you might want to do is get a portable tank and run the engine off of it and see what happens. If it solves the issue you now know it's not the motor and the problem is somewhere between the onboard fuel tank and the motor.

One other thing to check.. Some engines have rev limiters so if the engine exceeds a certain RPM it will cause it to sputter and slow down. Look up your engine model and year and find out what the wide open throttle (WOT) is rated for, it's usually a range like 5800 to 6200... If you're tack is exceeding the higher number, this could be your issue...

Good luck!