EricGorr

Super Power AssClown
Aug 24, 2000
708
1
New AMA rules regarding the YZ250F
Starting in 2003 the maximum displacement of a 4-stroke racing in the 125 class 250cc. But for this year 2mm overbores are allowed for modified amateur classes.

For those of you thinking about racing 4-strokes in the open or 250 classes. In 2003 the maximum displacement of a 4-stroke in the 250cc class is 450cc. The minimum displacement for a 4-stroke in the open class is 475cc.
:scream:
 

osheen

Member
Feb 27, 2000
202
0
Then your screwed and can't race at all. They probably won't even let you trail ride. They might even confiscate such a cheater bike. :scream: I don't know why the geniuses at the AMA and FIM left a void of 25cc in there. Probably a throwback from the 70's and the 2mm overbore limit. C'mon AMA, wake up, we don't have boreable cylinders anymore! The whole thing is so messed up. Currently you can't ride a '78 YZ100 in the 250 class but you can ride a KTM520. What sense does that make? Any smaller displacement bike should be allowed in a higher class. If you can beat the other guys on a lesser bike, more power to you..........
 

DualSportr

Member
Aug 22, 2000
527
0
Willie at DSP took the air box back , but could not believe we did not get an increase.

We found the same thing when testing it on the 400, then the 426 a year later. Each time we received the same response as you, surprise from the mfr. at our test results.

Tried again (at a customers request) on the 250, and got basically the same results.

But they look so cool!!!

BTW, our results with the ti exhaust system were considerably different than yours - hp was good, but torque numbers really dropped off the scale with our hydro dyno.
 

Pete Payne

MX-Tech Suspension Agent
Nov 3, 2000
933
38
Dualsporter,
Steve 125 and I had the same results with the carbon fiber air box on the 250F . And we got pretty much the same answer from DSP. They could not believe there was no better result anywhere across the RPM scale. We didnot put it on the track , as they claimed there was a much better response feeling on the track. But--- They do look cool. Oh well--Pete.
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
22,838
16,902
Chicago
Re: Do the math, where did 267 come from?

Originally posted by osheen
Where did 267 come from

Probably the same place that 420 instead of 417 (2mm over YZ400F) came from. After it gets repeated enough times I think it just sort of sticks. I guess I'm as guilty (or lazy :) ) as the next guy in this case .

I stand corrected. Thanks osheen.
 

jimt_yz400

LIFETIME SPONSOR
Dec 25, 2000
314
0
Pete Payne,
Cnn you share with us what you jetting specs were with the MR2 fuel (as well as elevation).

If we can get a decent improvement with just changing fuel why bother with those expansice bolt-ons!!
Thx Pete!!
 

Ando

Member
Apr 5, 2000
66
0
There is one more important option that should be mentioned. Thumper racing also sells a 4mm overbore in a plated cylinder. It specs out to 276cc displacement, which of course Thumper refers to as a 280 kit. Currently this is only available through Thumper, as they have some sort of agreement with Wiseco to supply the piston only to them. We have one customer with this set up, and so far it has worked pretty well. The only real question with this kit is, will the cylinder wall support this much overbore without any problem? Our customer hasn't had any trouble... yet.

The results were very impressive compared to my all stock YZF.
 

DualSportr

Member
Aug 22, 2000
527
0
If you guys want to get grumpy at anyone for playing the 'displacement game', how about Honda.

The one that comes to mind the quickest is their TRX300EX (quad) - it's really 282cc's. Talk about fudging the numbers!
 

utvols

Member
Dec 4, 2000
6
0
Why doesn't anyone talk about the 280 kit from thumper. I have a 01 WR250/280 that has run flawless for the past three months. This is not a sleeved cylinder and has not exhibited any of the problems discusssed about the 290 kit. I can't find any negitive to the kit and this bike has been harescrambled every weekend since it was put together.

Just a Thought!!!
 

Tex Labo

Member
Nov 19, 2001
1
0
Now, I may be wrong here....but I believe Thumper Racing also makes a 280cc kit. This eliminates the sleeving, and puts the largest piston possible in the factory cylinder.

You may want to double check me on this, but it seems I read a post over on Thumpertalk that mentioned this.

Just trying to help...

Tex Labo
'01WR250F
'01DRZ400e (For Sale)
 

jimt_yz400

LIFETIME SPONSOR
Dec 25, 2000
314
0
I wonder why more people haven't purchased the 280 kit? I'd think that this would be perfect for an off road (enduro/hare scramble) bike.
Ando, how much was the kit?
 

Ando

Member
Apr 5, 2000
66
0
My recollection is about $400 including plating, piston, rings, and gaskets. Thumper also has bored, plated cylinders in stock, so turnaround time is very quick. We would have gone with Eric, but he could not get the larger 4mm over piston, as I said above, that is only available from only from Thumper. I also tried to get it straight from Wiseco, but they said NO.
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
22,838
16,902
Chicago
Originally posted by Ando
We would have gone with Eric, but he could not get the larger 4mm over piston

He wouldn't have done it even if he could get the piston from wiseco (it's no problem getting them from JE in small batches). He and I have come to the conclusion that 4mm + plating clearance is too much to take out of that barrel and still maintain bore stability.

I wish you the best of luck with that customer.
 


Top Bottom