Best inexpensive ways to 'tame' a MX 2-stroke?

[RoostRider]

I have several new rider friends who have MX 2-strokes. (YZ250, KX250, CR125)

I would like to know what are the best inexpensive ways to make these bike more friendly to learn on?

They tend to ride them low in the power range right now, but I don't want to make mods that aren't reversable or expensive until they learn how to ride decent and can determine what they really want out of thier bikes.

This rules out things like new pipes and flywheels (but maybe not wieghts).... but things like jetting and sprocket changes are very much more what I was thinking.

Instinct tells me to gear down the two 250's, so they torque better and these guys don't stall so much on takeoff... any ideas?.. is there jetting for 'slow guys'?... lol

What should we be looking at for gear sizes? ('99 YZ250 and '01 KX250)

Any help would be greatly appreciated.

 

[ETS]

Most guys recommend flywheel weights and I've ridden a few bikes with them and they help the stalling. From what I've played with gearing it doesn't help a 250 that much. It does help stalling but for inexperienced riders it makes the bikes real snappy and might be a handful for some. Bad thing with fattening up the jetting you can run into fouling spark plugs and spooge all over the bike. I've got a flywheel weight on order for my rm250 so I'll let you know how it tames midrange hit these bikes have.

 

[rob129]

I agree with ETS, a lot of people do use flywheel weights to smooth the power out and make it more managable. I would not mess with the jetting, if its set properly leave it be.....you dont want to be in danger of seizing or have the back end of the bike covered in the dreaded spooge. You can also put a Pro Curcuit Pipe on, they smooth out the prowerband versus an FMF Pipe which is mainly for low end grunt.

Rob

 

[Chief]

RoostRider,

If you want a quick cheap reversable way to tame down these bikes, look into adjusting the preload on the powervalve. it's an external adjustment (takes about 60 seconds) and you will notice a big big difference in power delivery. You can increment the preload for your likes and riding conditions. It costs nothing. As you gain a better feel for the bike you can gradually set it back to where it will scream off idle.

I found that lessening the preload on my bike caused it to run a lot flatter and smoother at lower rpm's. It just plain runs slower and won't really take-off from under you.

If you don't like what you feel out of it, just take another 60 seconds and set it back to where it was.

I'm pretty confident you will be happy with the results if you take a little time to find a good setting for yourself.

Good Luck!!!!!!!!!!!!!!!!!!!

Chief

 

[Anssi]

A complete heavier flywheel for my YZ450F from Yamaha´s GYT-R range was about $100 plus maybe $20 for the flywheel puller from somewhere. That's not too bad considering how easy and effective this mod is.

 

[Chief]

Just one more thing, I put one of the heavier (maybe 13 oz ?) flywheel weights on my bike, and can barely tell the difference. The change it made on my RM was my-nooot compared to the PV adjustment.

 

[Vic]

You might try retarding the timing a bit.

 

[RoostRider]

those are some good ideas. Thanks everyone

ETS- I wouldn't even consider fattening the jetting out, as they already foul plugs. I hadn't thought about the problems with lower gearing. I will consider that before we make those changes. Please do let me know how the weight works!

Rob129- thanks for the input... some people get irked about people just agreeing with what was already said. I consider it to be evidence that the original poster was giving sound advice. (none of these guys can afford a pipe right now... :( )

Chief- I have never heard of that approach (just when I thought I knew everything... lol). Easy and reversable and cheap, JUST what I was looking for!!! Are flywheel weights as effective as a heavier flywheel? (read- do you get more weight out of a new wheel than you would from just weights?)

Anssi- I would have thought a flywheel for one of these main class MX'ers would be more than that. Maybe a flywheel isn't such a bad idea. The 2 250's are newer models and it shouldn't be an issue finding things like that, but the 125 is a '92, are there good sources for these bikes? Who is GYT-R? How do you gauge how heavy to go?

Vic- Thanks for the input. Cheap, reversable and easy.... sweet. I may be wrong, but instinct tells me that is a bad idea. I beleive that it would control the 'hit' of the bikes, but I think it would compromise torque as well (correct me if I'm wrong). These guys aren't as 'scared' of the bikes as they are just unable to ride them.. lol... That is to say that getting rolling is an issue, and riding them so slow that they foul plugs is an issue. I think retarding the bikes would exacerbate those issues.

Thanks for all the advice. Keep it coming!!... and help me out on my questions about your replies. EVERY reply is appreciated (unless your inclined to flame me for gettting some new ridders on the trail... :) )

 

[Anssi]

GYT-R, for all intents and purposes, is Yamaha, so get them at your dealer (or check them out under "Accessories" at http://www.yamaha-motor.com/ ). The one for YZ125 is $99.99 and the one for YZ250 is $99.95.

They are real easy to install, just get the puller. An impact wrench removes and retightens the nut nicely.

 

[RoostRider]

right on, thanks.... I'm off to look at it.. (I already have all the tools, and most of the knowledge, so it should be a snap)

 

[ETS]

If they are fouling plugs going "slow" then I would think about dropping the pilot jet down a few #'s. For instance on my 01' RM it had a 68 pilot jet. It would occasionally cold foul a plug and in general would load up during first/second gear technical riding. I put a #62 and it's much better. I haven't experimented anymore but I've gone leaner on the pilots of other 250's I've owned. Never had any trouble and they all run better. You can do wonders with a bike cleaning up the bottom end with pilot, air screw, and clip raises. I rarely change main jets because my riding style doesn't promote running 3/4 throttle and up. That is good info on the powervalve tuning but some bikes like the honda don't allow powervalve adjustments like the RM does. At least I just put a new cylinder on a 00' CR and didn't see how you could ajust it anyway. Hope this helps some more and my flywheel weight should be here in a couple days.

 

[RoostRider]

ETS- why the weights as opposed to the new flywheel for $100? Are they that much cheaper, or is it to refine the weight better?

I was already of the mind to lean the pilot out... probably just do it all and rejet the whole bike. They want to be super star MX'ers right now :) , but the reality is that this area has a lot more great trail areas than great MX areas, which makes me tend towards going more lean.

Needless to say, whenever we pass a sandpit, they drool, and they slam on the throttle whenever it opens up... and then lock up the brake for the corner... :) ... Michellin is going to LOVE these guys... lolololol... this makes me tend towards going less lean, but still leaning it up a bit.

anyways, the pilot jet is probably the one that is getting them right now.

Is this the place to ask what a good start point for jets is on these bikes or should I start a new thread on that one?

 

[ETS]

I think the weights for the yamaha's are just gytr parts and suzuki doesn't market performance aftermarket parts like yamaha does. Add on weights are the only kind I knew about until someone mentioned the gytr parts. If you do steahly searches on **** you'll find weights cheaper on there. Mine was $70 shipped to me off of ****. Supposedly used two times. Probably have a hard time finding a used one for a 125 unless it interchanges to a 250 since not many people off road the 125. I still would go with a steahly since I can swap back and forth easier and I don't have to get the puller out each time. I think most mx'ers benefit from a good tractable powerplant. Good luck telling them that but that's why the fourstrokes help alot of people become better riders because the confidence they build with hitless power.

 

[steve125]

RoostRider,

If you want a quick cheap reversable way to tame down these bikes, look into adjusting the preload on the powervalve. it's an external adjustment (takes about 60 seconds) and you will notice a big big difference in power delivery. You can increment the preload for your likes and riding conditions. It costs nothing. As you gain a better feel for the bike you can gradually set it back to where it will scream off idle.

I found that lessening the preload on my bike caused it to run a lot flatter and smoother at lower rpm's. It just plain runs slower and won't really take-off from under you.

If you don't like what you feel out of it, just take another 60 seconds and set it back to where it was.

I'm pretty confident you will be happy with the results if you take a little time to find a good setting for yourself.

Good Luck!!!!!!!!!!!!!!!!!!!

Chief

Chief ? lessening the pv preload allows the valve to open sooner. In all of my testing, that makes the RM hit harder!! I can make the RM run like a enduro bike by going an extra 1/2 turn "in" that increases the pv spring preload. Most of the hit riders feel with a 2 smoke is the pv opening, so delaying that, will also lessen or smooth the transition.

Also, most riders have very good luck with flywheel weights, to calm the hit and add traction.

It may be possible that you are lighting up the pv preload so much that the pv opens right away?? If this is even possible, it will kill any low end power, like a stuck open pv. The wrong way to go about smoothing the power IMO.

 

[Detonator]

1) A flywheel weight on a 125 is a must for off-roading, and a 250 MX'er needs to have more traction than spin by taming the motor and getting your suspension dialed in for woods. I race harescrambles on a 125, and can tell you that tractability and light weight matter more in a 2 hour plus race than horsepower or "hit." I run a 9 oz Steahly, which essentially just bolts on to the existing flywheel. Every bike I own will get a weight from now on.

2) The powervalve adjustment actually delays the midrange hit (valve opening) by a few hundred to a thousand RPM. A true woods bike doesn't/shouldn't spend much time at the 3/4 to full RPM range, just because traction becomes difficult with all that burst of power. If you listen to top racers at a woods event, they're playing in the 1/4 to 1/2 rpm range during all but open sections. The powervalve delay just helps to keep the motor from surging and spinning the rear tire. It really depends on the bike; a bike with an already strong low end transitions into the midrange less abruptly than a bike with a weak low end. I haven't touched my powervalve, and don't plan to. If you want, you can also play with "torque rings," which basically add 1/2 " to 1" to the junction of your exhaust pipe and the exhaust port of your motor. Apparently they can shift the powerband down a few hundred RPM.
3) Suspension and controls setup is different on a woods bike. A couple of hours of fast riding in rocks and roots with MX suspension and motor will have you wishing for some new knees and wrists.

 

[steve125]

Detonator, just a tip, the PV spring for the 01 YZ 125 is too light. Replace it with a 04 spring(same as the 00YZ) for a smoother transition into the powerband. I did it on my 03 and it worked great, took the dip out of the early part of the powerband, I thought was a jetting problem.

 

[Detonator]

That's the first time I've heard of that, Steve. Thanks for the tip! I've not noticed any dip, but I'm always game to try out a mod. I'll pick up a spring and see how it works out.

As for the woods mods, dropping a tooth on the front sprocket of a 125 is a must. You'll ride 90% in 2nd or 3rd anyway, and a lower first gear is a must for nasty spots if you like your clutch. I'd have to let the 250 MX guys recommend gearing for those bikes, but first gear on a stock MX bike is way too tall for woods. Real woods, I mean. Maybe a few teeth added on the rear of a 250...I'll leave that to someone else to advise...

 

[Chief]

Chief ? lessening the pv preload allows the valve to open sooner. In all of my testing, that makes the RM hit harder!! I can make the RM run like a enduro bike by going an extra 1/2 turn "in" that increases the pv spring preload. Most of the hit riders feel with a 2 smoke is the pv opening, so delaying that, will also lessen or smooth the transition.

Also, most riders have very good luck with flywheel weights, to calm the hit and add traction.

It may be possible that you are lighting up the pv preload so much that the pv opens right away?? If this is even possible, it will kill any low end power, like a stuck open pv. The wrong way to go about smoothing the power IMO.

Steve125, Yes, the PV would open prematurely and run flatter at lower rpm's.

You may very well be correct.

I am no expert and to be honest, don't feel like researching at the moment.

A few thoughts though. I am speaking of real riding experience and the bike was much more manageable for me in the woods. After awhile I noticed that the instant rev I needed for soft long hills with short runways was missing. Going back to stock cured the problem. The bike was very easy to handle with the spring tension set to zero. I cannot comment on performance with the preload set tighter than stock; I never had a need to try it.

If the hit is caused by the PV opening, then what caused the hit before most bikes had a PV? My guess is that it was the engine entering an rpm range where the horsepower curve spikes upward as in all two strokes. I am also under the impression that this is still what causes the hit.

The Powervalve was developed to increase power at lower RPM's. By keeping the exhaust port smaller, the compression is increased and so is horsepower. It also has something to do with port timing, uh, I'm getting even more hazy at this point. This is just bits and pieces of what I remember reading awhile back.

Other people here at DRN have mentioned that tightening up on the tension worked for them. In my case the bike still grunted up hills (not really trails but more like footpaths) well enough and never got a chance to hit the powerband. I never noticed that the bike hit harder, but maybe it did. Overall it just seemed sluggish compared to stock setting, which is ok in some conditions.

If you say that going 180 clockwise from stock makes it run like an enduro machine and smooths it out, then I'm probably going to try it for myself. That's if I get a chance, I'm currently trying to sell my bike for an enduro model anyway ;) .

Thanks for your experience and input.

Sincerely,

Chief

 

[magneto]

http://www.steahlyoffroad.com/

 

[youngr51]

You could but a larger base gasket on to decrease compression.Cheap and takes just a hour or 2.

 

[steve125]

You could but a larger base gasket on to decrease compression.Cheap and takes just a hour or 2.

True, thats also an inside tip from Team Yamaha to calm down the YZF 450, an extra cylinder base gasket along with a flywheel weight.

 

[ETS]

I finally had a chance to try out a 9oz. flywheel weight on my 01' RM250. I'm gonna have to agree with chief on the flywheel. Although I can tell it has altered the powerband it doesn't make a night and day difference. I can still stall it pretty easy but it is more luggable in the lower RPM's. The motor does seem to pull longer. Before it seemed like it ran out of revs in a hurry since it would wind right out quick but now it seems to have slowed everything down a little and it pulls longer. Might be my imagination though. If I was to do over I wouldv'e sent my cylinder to Eric for the money, sounds like he's about the best mod out there to taylor the powerband.

 

[Detonator]

The stalling issue is sometimes cured by raising your idle speed a tad. The flywheel weight likes a little more spin than the stock weight. A 9 oz weight on a 250 is a very subtle difference, mostly appreciated in slicker conditions or when the terrain becomes more technical. It certainly doesn't change the power characteristics of your motor, but for me it provided more useful power delivery, bite, and ability to wheelie more controllably over roots and obstacles. Areas where I need to "feed" the clutch (starts, 90 degree corners, sudden inclines, extra-slow 1st gear technical areas) are improved 100%, with no perceptible loss of response on straight sections. I also can find neutral more easily and miss fewer shifts, which is much appreciated, and something I never thought a flywheel weight would cure. I think YZ's come with a 7-speed gearbox...two neutrals, except when you want to find it!

 

[Chief]

I've ridden bikes done by Eric Gore, (one is mine) both done for different purposes, and both run well. (Much better than stock period) He can port for whatever you want and has experience with a lot of years/models. He helped me out a lot with mine.

Spending the $ on a pipe doesn't give as much result although it could compliment the work. The pipe usually costs as much or more than porting, and won't last as long.

We are still talking $ though. ;)

Chief

 

[wornknobby]

Cheif, what do you mean by setting the preloead? preload on what?