Placelast, Did you go with the Wr 4&5 gear? The problem I have is that I'm so heavy I need 13x49 or 13x50 for the tight technical hills and such and I still need a taller top that I can get with the above gearing. It turns out that I spun the rod bearing and I can't get any movement out of the crank, but the bearings should be replaced anyway. I was told by another D37 racer that the plastic inserts on the inside edge of the YZ250 crank will restrict gasses/oil flow at extended high RPM's. Those units are designed for MX and boost hit off the bottom, but cause many desert racers to burn up the bottom ends when running wide open for so long. I was also told that from now on, every now and then hit the kill switch for a count of 2-thousand with gas full on when running wide open on very long runs. This feed additional fuel in to the system quickly and keeps the motor rich enough to handle the high RPM's non-stop.
On the motor I was advised not to over bore, due to quicker wear and that coming off 4-strokes, I just need to learn how to ride a 2-stroke again to control the motor and not burn it up. I'll ask Eric Gorr though. On the porting, I was told that the 1999 YZ is very go stock and to not do anything to open the exhaust. Ty Davis is porting to lean on the bottom by 2mm or so and that is working well. I'll have to look into that more. I guess one problem with the big bore is that porting will be need for the system to work.
Bottom line, I'm going to call Eric and tell him how I ride, what I ride, and how the bike is setup now and see what he has to say. Sounds like you had good luck with yours. I hope I have the same.
Thanks for the input.
Ed