Smooth out YZ 250 power

hmmm, i would have said a flywheel weight, but you already did that. i honestly do not know, but maybe you could fasion some kind of "restrictor plate" out of masking tape in the airbox. :think:

Hey bud. I have the same bike. Converted it to ride the GNCC series. I love the power in the sand, but it is a bit much off road, or in slippery conditions where trees and roots abound. The SST pipe is known for being a top end screamer. I'd put the stock one back on...or try the FMF fatty version which is more of a low end pipe. I certainly notice when the top end is fresh, the bike also has more bottom end torque. They sell torque reeds also...not sure if that would be compatible with your current set-up. I also added a Clarke 3.0 gallon tank...the extra weight seems to of calmed it down a bit! My favorite mod is the Scotts damper which also helps keep you out of trouble. Best of luck, hope this helps!

;)

Crusty,
Try retarding the timing 1 or 2 degrees, it makes a big difference. If you are interested in porting, Eric Gorr's Mo Better porting is a great choice for the woods. :D

Scott

That SST pipe is for experts(I.E- TopEnd Screamer!)
A Pro Circuit pipe and shorty silencer will make the smoothest powerband you could imagine!

Hi Crusty1,

Sorry for the delayed response.

I had Eric do a 275cc kit on my 99YZ250, added an FMF Gnarly pipe (just a fatty pipe with thicker gauge material) and a flywheel weight. I asked for something with a very KDX like power delivery, as 4-stroke like a possible. He listened closely to my wish list, answered my questions giving me the pros and cons of each option and delivered exactly what he said he would. Eric sent the cylinder out to US Chrome where it was treated to their super tough NICOM plating. Eric removed some material from the bottom of the cylinder to lower the port timing/height and modified the head. Like you I was looking for the smoothest possible power with lots of bottom and mid grunt, essentially an easy to manage big bore version of the KDX’s power plant. Basically that's what I got.

The 99+ YZ250s are already known as one of the smoothest MX power plants out there but I was looking for a smoother woods version. The power deliver is very linear with lots of usable grunt everywhere and noticeably more than stock power plant. I think the best way to describe it is to imagine the stock dyno graph for the bike. The big bore, cylinder and head work basically shifted the power curve down (maximum at a lower rpm) and to the left (more power). The engine produces more power everywhere but the power drops off a touch sooner with less over rev, it's a big bore after all. I can now easily lug the engine anywhere to snake through the tighter trails and just upshift if wheel spin becomes a concern and grunt my way out of it. As promised it's very 4-stroke like. I'm new to MX riding so really appreciated the smoother power on the track as well. The traction out of corners is excellent and the power is easy for me to manage every where on the track. Even on really hard surfaces I was getting as good or better bite than my buddies YZF250 and the engine produces a seductive 4-stroke like bark at lower rmps.

The YZ wouldn't be as easy to ride in really slick of tight conditions as the KDX would, its still a real power house that likes to be ridden quite aggressively, but Eric produced a very well rounded power plant that makes the bike not only much more user friendly but really versatile. The only down side is that my grin is so big my helmet will hardly contain it, I'll have to order a new one soon.

I'd encourage you to give Eric a call and talk to him about porting your stock cylinder and modifying your head to smoothen out the power vs. the big bore kit. I can't imagine getting something this 4-stroke like with out the big bore but he'll certainly be able to walk you through the differences between the two.

Like others have said the SST pipe was intended to make the power delivery more aggressive and I can't see it being a good choice for your application. I'd certainly give the stock pipe a try and see if it doesn't smoothen thing out for you a little.

You might consider adjusting your throttle position sensor? You can adjust the plate so it advances or retards the midrange enrichment to work best for the throttle response that feels best to you.

Hope than helps,

David

I'll second Canadian Dave's advice (great report Dave). Eric provided me with basically the same package for my KX. It's awesome in the woods and elsewhere, I just don't ride the elsewhere very often. I know that grin Dave refers to. ;)

bump

I agree, dump the SST for the stocker. The SST is supposed to be a topend pipe. Try that, and get back to us. Also, you may have to do a little jetting with the pipe change.

you can buy an enduro throttle that may help keep accidental power hits to minimal..on these throttles, you can turn it more before its wide open.
i suppose a bigger front sprocket could help as well. do you think if you had gotten a 13oz flyweight that it would have been better?

oh, dont forget the reed spacers and power valve cover spacers,, that may tame the hit a bit too.

Did you play with the gearing a tall? I was having a heck of a time keeping my rear wheel hooked up on a very slick, frozen track. I went up 1 tooth on the countershaft and the bike became much easier to deal with. I doubt it is as god as some of the other mods, but it was only 8 bucks. My 1997 easily has enough torque to pull this gearing especially since the other guys talked me into trying the short powervalve cover.

I just had Eric do he Mo Better on my 94 YZ250. I have yet to even break it in yet but I will fill you in on it when I do. I also went with the 11oz flywheel weight ,Boyeson dual stage reeds and a FMF Gnarly pipe with a TC-2. I went with lower gearing as well and ride one gear higher and lug the engine a bit. This engine tends to have a real nasty midrange hit which made it a handfull in the woods.

Steve

you all thats saying the sst is an top end should look closer
its an improved version of the fatty more power every where
deal says so in the pipe comparisons in the fmf catalog

Yes, it does, but FMF will tell you everything you want to hear.

i would keep the SST, from what I have seen, it adds power everywhere, bottom and top....it works very well with erics low to mid job also

I guess I shouldn't be going off on subjects like this, I don't have a YZ and I don't have an SST. Bruce (bclapham) would be a lot better person to talk to about YZ's since I see that he has done quite a bit of tuning with his... But one more thing, crusty1 is looking to smooth out the power on his bike, is the SST a good pipe for that? Also, congrats on the new avatar, it looks sweet man!