Ok I need help. We burned a hole in the clutch side piston a few weeks ago. There was no scoring on the piston or cylinder just melted right through the top. I'm told this is from pre-detonation. We rebuilt the engine and the other piston was perfect. The same cylinder still seems to be running lean...the plug is a whitish/grey. I'm running it at 900ft / 20-30 degrees f. I have 310 jets in and the needle is in the center clip position. Gas is only 87 with oil mixed at 100:1. Plugs are BR9EYs. Plug seems fine at idle but hot if I check them after a midrange or top end run. The jet chart on the sled says I should be running a 270-300 jet so I shouldn't be running too lean and can't understand why it's only happening on the one side. Do I need to jet up, move the needle, change gas, change plugs or give up. Carbs have been cleaned several times so I don't think it's a plugged jet. Thanks.