ORIGINAL: pe reivers
The ignition seems a little too advanced. Try to retard slightly until the soot on the plug electrode climbs right up to the tip.
Today I flew w/ the Stihl 50:1 mixture and I switched from 89 to 93 octane gas. The higher octane contents has the same effect as retarding the ignition. I have to take the cowling off for some repairs anyway so will retard the ignition a bit as well. The engine runs great btw on the higher octane. I'm running my DL w/ an RE2 pipe and the engine sounds like it's running high idle hovering the 18lbs plane.