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Discussion Starter · #1 ·
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.
 

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What year?
I am pretty sure you should be using premium pump at minimum 91 octane.
My 02 runs 320 jets in the center notch. Mine burned down the same way and it was way lean on jetting( done before I got it). I checked timing and I was advanced about 2 degrees so i adjusted the knob accordingly. Make sure your timing is at TDC. If it is advanced, it will also cause detonation. You will need to adjust octane according to your timing.
 

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Sounds like an air leak. If you have checked everything like carbs, carb boots and flanges etc., then the next step is the mechanical part of the engine. Has the engine ever been over rev'ed / broken belt at speeds? The engine needs to be charged with about 5 psi of air and sealed to look for a case seal leak, or just go through the lower end.
 

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Discussion Starter · #4 ·
Sorry the sled is a 2001. I relplaced the gaskets all the gaskets between the intake plate and the block. The boot looks fine to me but not sure what I'm looking for. I can't tell you if the sled has ever lost a belt at a high RPM. The sled is a backup sled of my buddies and I was the lucky one who jumped on it and burned it down after 1 mile...hence I'm the one rebuilding it. I've been told that a lazy mans way to check for an air leak is to spray a little carb cleaner around the outside of the case seal and or intake seals and listen for an RPM change. Good idea/bad idea?
 

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Intake cleaner is a good thing it sounds like a crank seal, or a piece of dirt in that carbs check the setting of the air mixture and fuel mixture and the synchronization.
 
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