I calculate a little over 110 RPM. You do know that this engine rarely hits above 9,000 RPM don't you? The prop is right in the middle of the range suggested. I would look at putting in a larger or stiffer fuel pick up line. I have had the same thing happen and the soft flexible line was colapsing, when the engine slowed down the line would open up, causing the engine to surge. It could also be caused by trash in the carb or needle. Or the fuel lines are too small.