Finally got the engine worked out . Problem 1: I had a failed head gasket. The engine wasn't using much water, but the compression was low. I tore down the engine to replace the stock piston and found a water trail across the head gasket. Turns out a couple of the studs were loose in the cylinder. I put a standard Wiseco piston back in; ring gaps were both 0.013" out of the box and piston to bore was 0.0014". Everything looked great. Freshly assembled, the engine was making 160 psi cranking compression (~200 psi at sea level). Before the tear down, I was only getting 120 psi (~150 psi at sea level). Problem 2: The power valve linkage was leaking into the crankcase making me think I had a blown crank seal. I had to use a tire pump to get 6 psi in the engine and it would leak down immediately. I pulled the linkage apart, installed new bushings and greased the linkage in the case and in the cylinder as per the manual (I globbed Moly 60 paste all over the pinion shaft and lower bushing). When I got everything back together, the engine holds 6 psi with no leaks for 3+ minutes (I didn't wait the full 6 minutes). While I was in there, I put in a VF3 and milled the head 0.020". Unfortunately, the Wiseco piston is about 0.010" shorter than the stock piston, so my squish clearance only dropped from 0.060" to about 0.050", so I'll pop the head off and take another 0.010" to 0.015" off soon. First ride was today. BOOYAA! Now THAT'S how a 250 2-stroke should feel. Excellent power right from the bottom to the top. I think it's down a bit on total peak power compared to my ported '03, but the usefulness of this engine is just outrageous; and that was with my stock 13/48 '03 gearing. Even at rather low RPM, crank in some throttle, and it just goes; with enthusiasm. Don't think I need it, but I might try a 50 out back soon. I'm still having trouble with carburetion. I'm running a 45 SJ, A735/289R needle in 2nd clip (2 steps larger diameter than stock) and 165 MJ. Idle is good, WOT is good, but everything in between is suspect. The jetting is still really fat from off-idle to about 1/4 throttle, then lean from 1/4 - 1/2 throttle, then pretty good. According to JD's Jetting Guide, a 42 SJ and the Yamaha N3EK needle look like a good alternative to address my rich/lean issues. The verdict is: If you have an '02 - up CR250 and really want to make it crank like a YZ250 (granted, some don't), don't waste time porting the stock cylinder. Pop in an '01 engine go have fun .