Thanks to both replies... I will look into your suggestions... I am running a 12X6 with a Syn/Castor oil and I added 5 oz castor to the gal of fuel...Plug is idle bar and not sure of make... Again, thanks.. It would not hurt for me to put a new plug in and try to see if that stops the surges at the high end... Understand the 10,000 rpm... will go with bigger prop also.... Richard