Help! What caused this scratch on the piston?

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I would expect more wide spread damage from sand, that looks like a piece of the engine came loose and made its way between the piston and cylinder.
 
Well i flushed the case and found nothing. All bearings and parts seem to be in perfect condition. I contacted Doug at ESP to see if he could offer any insight
 
Im gonna have to go with J since i couldnt find anything broken. C-clips, bearings, and everything else looked fine. Barely even any wear in the cylinder or piston other than the single mark on each

Its still crazy to me that that little damage and it wouldnt even start or even try to start. My RCMK had way more wear before it quit
 
Talked to Doug at ESP and he said its a lean running issue. Plenty rich on the low needle but too lean on the high. Im guessing he has seen this plenty of times as he described the damage i would see exactly without even seeing the pics. He says he can cut me a deal on i knew head kit if i decide to rebuild it.
 
I have two ESP mills now and hope I never need Dougs help with an issue, but it's good to know he's willing to help a guy out should problems arise.
 
The problem is if you are going fron dirt to sand especially with paddles on. It lugs the engine more in the sand because of the bite it has. Usually with dirt you are not bogging it as bad as you do sand. So you will need to richen up to not run into the issue you had because of the additional load on the engine.
 
Well heres an update and more issues. I got the gizmo full mod head kit and finally had time to get everything back together and installed. I installed the 990 carb and finally got the engine to fire. Problem is with the needles set at factory settings, the engine starts and revs really high even with no throttle input. I checked the intake and carb gaskets for leaks and found no issues. Even tried a second carb and same thing. Any idea as to what the problem could be?
 
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