1992 YZ250 Cylinder Repair

Discussion in '2-Stroke Machines' started by Kurlon, Oct 28, 2021.

  1. Kurlon

    Kurlon Well-Known Member

    I've got my ye olde 1992 Yamaha WR/YZ 250ZD smoker that I've been abusing and racing for quite awhile now in supermoto trim, with a brief couple of tries road racing. It's current setup is a 6mm overbore that is linear as all hell, but I'll be damned if I can keep it from detonating in 4th and above. In the mean time I've got an orange infection in the garage that has me on a modern 500 for road racing, a 150 for supermoto, and a 250 woods machine just for good measure, but I can't let go of the Yamaha after all this work so it needs a new purpose in life. Seeing as the geometry is setup to be almost TOO stable in supermoto trim, I want to prep it as my rain road race bike.

    So, as noted it's a 6mm overbore, mated to a wide ratio transmission. I can get decent perf out of it on a short track where I'm bouncing between 2nd and 3rd, at Loudon it struggles with the gaps between gears. The massive overbore also means the powervalve is basically ineffective. My 'plan' is to build a new motor on a YZ250 bottom end I have, that'll clear up the gear spacing issues. I've got a stock bore cylinder that needs repair, and a new old stock powervalve with no wear. I'm just missing an untouched head to complete the 'kit'. What I'm curious about is who to use to repair, replate, and ideally port the cylinder and work the head to match. Is there anyone that still has experience with the prior generation Yamaha smoker?
  2. 90kacoupe

    90kacoupe Novice seeking Help

    Your best bet would be to work with somebody like Gary Braun or Scott Clough to get the porting and head to work together. I'm sure your detonation issue on your current setup can be resolved with a well shaped combustion chamber with the correct volume for road racing.

    If I can make a bullet proof 45hp road race motor out of a 1971 air cooled enduro engine, That WR/YZ will easily do it.
  3. Riot

    Riot Member

    Hey Kurlon!

    Millennium technologies can handle the bore and plate. I think Gary Braun (mentioned above) is working with them now as well.

    They recently did a bore and deck job on my even older MX250 and the service was top notch.
  4. racer_11

    racer_11 moto nyne guy

    power seal!

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