Originally Posted by Sport_Pilot

It has been a long time since I have done much tuning of a four stroke. The OS F is popular because it is one of the hottest 4 stroke plugs out there, but unless you are using very low nitro fuel it is too hot for the best performance. It's also not the hottest, the Thunderbolt 4 stroke plug was the hottest plug I found and perfect for no nitro or 5% nitro. The OS plug is good for 5% and 10% nitro. The next hottest is the Fox miricle plug and good for 10% and 15% fuel. The coldest was the K&B 4C which is good for 20% and higher nitro. All gave a good idle though the K&B did not on the lower nitro fuel. I had no luck using no 4 cycle plugs including Fox idle bar and Enya #3, they lost too much heat at idle and could not get both a low idle and good transition, but other engines may work fine with them. I used these on an older OS .91 Surpass, .91 TT, and a .30 Magnum, but only an OS and Fox four stroke plugs in the latter.