Toning down a YZ250

[grumpy]

After reading some other threads, I see there are some very knowledgeable people here. So it seems a reasonable crowd to toss a question to.

I have an '83 YZ250, have had it for years and love it. What I'd really like to do is tone it down and make it a better trail bike as that's 95% of what I do with it nowadays anyway. The suspension still needs some work, but I've got a handle on that. I can't find where flywheel weights are available or different spring rates for the power valve (doesn't use springs anyway). I guess the timing could be messed with, but that's an area that I know very little about. When I got the bike it was geared taller than stock, it was at 14/48 (stock 13/48) and I re geared it lower to 13/50. I recently rebuilt the top end on it and found it to be .080 over but ring gap is at the minimum spec so it's tight. In the process, I changed the stock reeds for TDR reeds and I've jetted the stock carb. It does have a DG pipe and silencer and I've added a cobra spark arrestor. I've noticed the '84 has other more modern pipes available, but this one the pipe runs off the left side instead of the right so a bit of modding would have to be done to change to a different pipe.

It runs really good and has a really strong punch in the mid range. I'm mostly curious what, if anything, can be done to broaden and perhaps mellow out the power a little more for a better trail/woods rider on this thing.

[Chokey]

Early '80s YZ250s were known for short, punchy, abrupt power curves. On my '86, I had the cylinder base turned down, the exhaust port widened slightly, and the transfers modified to re-direct the flow more towards the intake side of the cylinder instead of the exhaust side like the factory configuration tended towards. In combination with re-chambering the head and a slight reduction in crankcase volume, the low-end was much improved with no loss in the middle or on top.

[evo550]

Grumpy,
You could try adding a second base gasket to the cylinder, this was an old trick to taming 2t 500's by lowering compression.
Cheap to try and can easily be changed back to standard if you don't like it.

[grumpy]

Interesting idea Evo. I'll have to give that a try.

Chokey, I'm sure your ideas will work. Unfortunately, I'm up in the sticks and there's no trustworthy shop to perform such work. So, I'll wait 'til next time I do a big tear down and consider sending it off for that kind of work.

[dogger315]

Some more bolt on tricks that will tame the power a bit.
Install a flywheel weight.  That will smooth the hit and the
increased flywheel effect will help in slow technical sections.
Another bolt on is replacing the aluminum clutch plates with
steel.  Produces essentially the same results as the weight
but to a lesser extent.  You can add a second head gasket.
This will lower the compression ratio a little and result in a
little less power.  Finally, changing the gearing from 14/48
to 13/50 only makes matters worse if you are trying to
lessen the hit.  Reinstall the 14T CS sprocket. That's like
removing 2 1/2 teeth off the back sprocket and will mellow
the hit.  You can even try a 46T rear sprocket if you want
more.  I used to run a 14/46 combo on my CR500.  After the
start, I could practically race the whole moto with two gears!
 
dogger