MOPED ARMY

some more photos:

Pre mix and oil injection?

So that is a stock piston that was subjected to who knows what while running on stock injector? Stock injectors on these are fine for stock application as long as they dont run out of oil or the injector hose does not come off the carb. (edited)

That's not debris damage, that's a four corner seizure. The underside of the piston crown is burnt so you know it was getting too hot.

70 main jet is too small with MLM pipe.

"My guess is that it was running lean partly because of running premix with the oil injection still working in combination with poor carb settings."

If you were running premix and oil injection, you were running even leaner because more oil is taking the place of gas. Gas not only powers the engine but cools it. Too little gas means not enough cooling.

More oil is not a good thing - especially too much oil.

Your temperature of 378F is not necessarily a deal breaker for a properly jetted bike but since you were way under jetted and running too much oil, that probably led to your soft seize. Qt's also tend to build up temperature as you go up even slight inclines. You are always fighting heat build up on those damn things.

You need around a 75-78 main jet in the stock carb for stock cylinder bike running an mlm pipe. Also some of those PW heads are high compression heads. Probably not the case with yours since your compression was only 110psi.

Here are my latest findings with the qt. If you open the exhaust port completely and jet it right, it cools down real fast at full throttle. Mine would tend to get hot at idle after coming to a stop. I went to a slightly bigger main jet which should help (SHA carb on this one so main jet affects idle).

I did not, I plead ignorance.

pw50 cylinder = qt50 cylinder. Same goes for rings, piston and head. Same engine. No mention of any piston chatter which might suggest clearance between piston and cylinder was too great. If piston to cylinder clearance was too little . . . I guess it's a possibility but these pistons and rings are interchangeable between the pw50 and qt.

failure to up-jet from stock main jet when running performance exhaust and higher flow air filter led to seizure. (edited)

ever so slight scoring on the piston suggesting what was to come when you put a significant load on the engine up the incline.