I had a pretty bad detonation problem with my 512 stealth headed motor 11.3:1 comression. I changed the power valve from a 3.5 to a 5.5 upped the jets by 4 dropped the heat range on the plugs down 2 and got the corn ( ethanol) gas out of it and put pure gas in it. The detonation problem went away. I think getting rid of the corn gas and putting the colder plugs in it made the biggest difference.