I would first verify your fuel mixture in the problem RPM range. I would check for air leaks between the carb and engine, you can use a smoke stick to check. If no leaks maybe try a pilot jet change to see if that helps, see what sizes are stock and make notes of where you started and all changes. Will it run at lower RPM with choke on or off, if on you may need to go richer, if off you may need to go leaner. You could also put in a new spark plug and run it for a while at the lowest RPM possible then check the plug color, white would indicate a lean condition and dark would be rich. A very hot exhaust pipe wqould also indicate a lean conditionOptimum would be tan. Good luck
