1989 Cr250r - rebuilt last winter with a knock-off piston as this is just a casual bike. It had blown up due to waterpump failure. Replaced everything, got a new cylinder that still had hone marks off ebay. It went through its break in fine, with 5 heat cycles. Tons of power. Took it on a 15kms run down the gravel road - great. Rode it to my buddy's place 5kms away and as I was getting there power faded fast, soo I was in 5th gear gunning it and going a jogging pace. when it shut off it would not fire again. Pulled the plug and found flakes of metal. since I melted the last piston and had gobs of metal on plug I assume I had wrecked the piston and possibly the cylinder. pressure tested and it was at 65. assumed it was lunched and parked it. That was 9 months ago. I finally got around to taking off the head (I said this was a casual bike) and the cylinder walls look good, no scoring. Put the head back on and pressure tested it. 145. hhmmmm. Without taking the cylinder off to look at the piston and rings can anyone think what might be happening?? The air screw had been stripped and so was running lean I know but too lean?? This was a bike that ran good two summers ago and I have not changed setting besides the air screw (I had srcewed it all the way in, they tried to back it out the 1 1/2 turns but it jammed and snapped off the slot. So what I am asking is if I could have started to fry the piston - could I NOT have scored the cylinder?? could I somehow be getting 145 psi with a partially fried piston??