ERIC: Is this possible...or am I just dreaming?

[crusty1]

Eric:

I've got a 00 YZ 250 with stock pipe (had SST put stocker back on) Boy. power reeds, and 11 oz flywheel wt.. I came off a KDX 200 wanting more power...boy did I get it. I ride mostly tight woods but have a 1/2 mile moto track on my side yard (kids race/I pretend).

Love the suspension, handling, etc. but need to smooth out the power or else I'm looking at buying a 02 KTM EXC 250. I really prefer everything about my bike over the KTM (nothing against KTM, we've got 2 of them in the garage) except the power characteristics.

IS IT POSSIBLE/COST EFFECTIVE TO TRY TO ACHEIVE A POWER BAND ON MY YZ LIKE THAT OF A EXC 250 (ALMOST LINEAR)? OR SHOULD I JUST SUCK IT UP AND BUY THE EXC?

ANY HELP WOULD BE MOST APPRECIATED!

DENNIS

 

[bscottr]

Crusty,
I had a similar situation with my KX and had Mr. Gorr port it (Mo Better). Power starts early and pulls very linear through the power band, much like a 500. :) No or very little hit. I highly recommend Eric as well.

BTW Until you have it ported you can retard the timing 1 or 2 degrees to ease that hit.

Good Luck,
Scott

 

[EricGorr]

Thats funny you mentioned this about the YZ. I built a top end for Dave Otteson (Canadian Dave) in the KDX forum. We built a 275 with a lower compression ratio (top end) and some epoxy re-shaping of the transfers to reduce short circuiting of fuel at woods riding speeds. I also lowered all the ports by truning down the cylinder base (.5mm, .020 inch) and cut the squish clearance down to 1.25mm, .050 inch).
Dave bought the bike used (IE beat bad) and finished out the season. The top end was worn meaning that the piston rings wore out the stock coating. Also the center powervalve was scoured badly. The reason was that the stop-plate wasn't rigid enough and the flat plate on the valve cracked allowing the valve to wiggle in the valve bore of the cylinder. The YZ uses a 2-stage powervalve with the center opening first. Dave's valve was siezed so the valves weren't opeing correctly which made the powerband stahl before hitting when the secondary valves opened. Yamaha has since corrected the problem in 2002 with a new stop-plate and flanged Allen bolts.
For more information about how this bike performs, check out the KDX Forum.

 

[Rooster]

Eric, is there anything I need to do to my '00's power vavle to avoid such problems?

 

[EricGorr]

You can install fender washers under the two bolts that fit through the stop-plate that covers the center exhaust valve. The stock flanged screws aren't large enough and the sheet metal stop-plate cracks at the bolt holes.

 

[MikeT]

Eric, I think your answers need to be more informative. The least you could do is give some details.... :confused:

Anyway, Do you think adding an extra head gasket would help mellow out the power a little for him?

 

[Canadian Dave]

Sorry Dennis I missed this thread and fired off a reply to the CR,KX etc forum. You can find my response to your question HERE

If you need more details just say the word, I'd be happy to get as specific as you'd like. I love this motor :)

I've ridden an EXC250 in the past and can tell you this engine is certainly more 4-stroke like than the one I rode.

David

 

[crusty1]

Eric/Dave:

Thanks for the guidance/info. I almost traded my bike for an MXC 250 the other day before I read your posts. When I rolled my bike out of the van the sales guy said "cherry bike." I take that as a good thing and decided to keep my bike as the MXC looked a little beat for an '01. I'm going to look into the big bore/head mods/mo better porting to find out about turnaround time. Here in the Northwest you've got to seize the season for riding so I might wait until the rain returns. Around here...that won't be long!!!!


Thanks for the very excellent, and extremely detailed, information. It's just what I was looking for.

Dennis

 

[bclapham]

OK, ive just done the thread run around, and it looks like there could be a potential problem with the YZ powervalves.

Ive got an 01 YZ250 and i assume i could also run into the powervalve damage (described above) at a later date. is this modification difficult to do? is it just a case of taking off the powervalve cover and adding the washers, or can it only be done with the top end off?

if anyone can lend a more precise description of the fix i would be very grateful, since i have never had the top end off the bike its difficult to envision what you guys mean!

thanks in advance

bruce

 

[bclapham]

hopefully if i add this you might be able to help me easier.

http://216.37.204.205/Yamaha_OEM/YamahaDB.asp?Type=13&A=203&B=4

the powervalve (#8) is held down with the plate (#9) using two screws (#10)

i think you are saying to put some wider washers under screw (#10), correct?

can the powervalve cover be easily accesed whilst still in the bike?

thanks (again) in advance

bruce

 

[EricGorr]

I think that you have to remove the cylinder in order to gain access to the front powervalve cover.

 

[kev_rm]

Same problem on my 99 RM250. Is it worth waiting for a top-end rebuild to be needed, or should I just send it in for mo-better and do a rebuild prematurely?

Speaking of which, how often to rebuild for woods riding? 10-15 hours seems a bit often.

 

[bclapham]

thanks eric! my bike isnt too thrashed so i will wait to do this little mod the next time i do a top end, probably after the summer. i will get in touch at that time to talk porting etc.

 

[Rooster]

Thank-you Eric. I will do like bclpham and wait until I do a top end in a month or so.

 

[Canadian Dave]

Eric I’ve had a few PM’s asking some questions about the work you did on my top end and the end result. I’ve suggested they give you a call directly but I thought others might be interested in your response as well. When I talked to Eric about doing my top end my main goal was to achieve the smoothest possible power delivery he could, make it as 4-stroke as possible.

Here’s the question:

Dave I've just read over the “Is this possible...or am I just dreaming? “ thread in the Performance/Mods Forum and I have some questions for you. . . . . . Typically when looking at a list of improvements for increasing bottom to mid performance one of the first modifications to the top end is to turn the head down on a lathe and bump the compression. Eric's response indicates the compression was lowered, why? . . . . . . . Eric also talks about cutting the squish clearance down to 1.25mm. I'm assuming this is less than the stock setting. What effect does this have on power delivery?

Thanks Eric,

David

 

[FDR492]

You're kidding me
The power cant be that bad getting off a KDX.....they are FAIRLY comparable. Hmmm... they say that if a 250 had the powerband of a 125, you'd have to sit on the bars to keep the front end down :)