YZ125, How to smooth out the power?
 

I was hoping maybe someone here had some suggestions or ideas. A friend of mine has a 2001-ish YZ125. The bike runs great but hits really hard. She only weighs probably 100 or so lbs. it's just to much for her to try to handle. Do any of you have any experience building one of these? I think if we could smooth out the hit and bring the bottom end power up a little it would be perfect. Sacrificing top end shouldn't be a problem. I was thinking of tinkering with the power valve and maybe lowering compression. Ideas?

Fly wheel weight. Put the heaviest one you can find. It will help tremendously.

Make your 125 power smoother and easier to control with a Steahly flywheel weight. Soften the hit and make it more chuggable. Most woods riders will benefit as well as all beginner and timid riders. Easy bolt on installation, no drilling, grinding or welding. Consider getting the engine lock up tool (E-3) to make installation and removal even easier.

http://www.steahlyoffroad.com/yamaha...-weight-2.html

Fww, hasslebri has a very nice yz125 with a heavy fww and man is that a woods weapon. Lugs everywhere. I am sure jetting helps as well.

A flywheel weight would be my number suggestion but you can also use an auto-clutch to help calm it down as well.

I have an 2005 YZ125 that I trail ride and race, I'm not a buck-o-5, far from it.

For a 125 mine is pretty tame, no massive hit when it come on the pipe, however it will haul the mail with my big a$$ on it. By the way, I'm 51.

The stock pipe is one of the best all around pipes, not sure about the 01-ish model...but I use a OEM pipe.

I do NOT us a flywheel weight, thought about it many times...but never pulled the trigger.

I do have a V-Force reed cage. A must have IMHO, nice difference over OEM, help throughout the power spread.

I use 3 different silencers, FMF TCII (Spark Arrestor) Leo Vince ( Shorty w/ Spark Arrestor) and OEM for moto. The Leo Vince offers the best sound and all around power, however it is a pain to rebuild.

Gearing, I run 13-48/49 & 50, 13/48 is OEM, 49 is really good, keeps the bike out of first gear, more in the meaty gears 2,3 4th...hoverever your results my vary if your tranny is a 5 speed. The 05 model has a 6 speed gear box. 50 tooth rear I use for tight woods.

Carb mod - I feel that this was by far the BEST money spent to make the bike more user friendly. http://rb-designs.com/rb_designs_llc_011.htm
I was able to try this mod back to back with the mod carb and OEM carb and the mod carb was better all around. The divider plate really helps with the bottom, smoothes everything out, no suprises.... I had Ron perform the entire mod, along with his suggested needle and taper bore to 39mm.

Jetting - I run a 50/50 race gas / pump gas. I am jetted 420 main, 40 pilot, RB Spec needle (This is NOT a ONE off Needle) This needle is a OEM needle from another brand 125. I have tried a 37.5 pilot jet, this wakes up the bottom, makes it very, very crisp and produces a much harder hit. For offroad I prefer the 40 which is OEM setting, but combined with the RB Carb mod really make the bike easy to ride. This make it pretty easy to "short shift" the motor and still keep it in the meat. I like the 0-1/2 throttle a little rich, which tames down the hit. If you go to the video page there is a couple of clips on me racing moto on my 125, if you listen close you can clearly hear that my 0-1/2 throttle is jetted fat, there is a section of the course where a have a almost 180 turn, I get down into 1st gear, and the bike pulls nice through the gears, but you can also hear a little brubbling from the jetting....but this is how I like it...for the very reason your trying to avoid.

I don't think there is much you can do to the power valve with regards to adjustments....I would not jack with the compression, or try adding more base gaskets. The OEM parts manual list only one thickness of gasket.

RB can do a squish mod as well, refer to his website on instructions. I also have this mode performed, howver it was done by another shop.

I would not rule out the fly wheel weight idea, I have a buddy who has a 12oz on his 125 YZ, he tried the 9oz, (which he wants to sell me) he stated that the 9oz was not enough for his style, so he went for the 12oz. He says it helped.

Hope this helps...good luck

Wow, thanks for all the input! I really appreciate it!

I hadn't even though of a flywheel weight. I think that, and fatten it up on the bottom would probably be a great place to start. I really like the idea of the RB carb mod too. I'll just have to see how much they are wanting to spend.

Thanks again!

On my rm125 98 I added an 8oz steahly fww for stall resistance, vforce3, fmf fatty pipe and shorty muffler, leaned out the pilot circuit, and went to 13/52 gearing. Biggest difference was the vforce and jetting when it came to power delivery but the fww helps so much in single track with stalling and lofting the front wheel that I reckon it's a must as well.

The FWW will do a fine job to smooth out the power to reduce the hit and help prevent the bike from stalling, but a FWW will not add low end power.

Part of the reason the hit on these MX bikes seems so drastic is partially due to the lack of low end power. I did some similar mods to a YZ85 for my son to ride in the woods. One helpful modification is similar but opposite to what GG riders do to increase top end power. This modification raises cylinder by adding base gaskets to delay port timing and move the power band higher into the rpm range. Conversely, you can move the power band on these MX bikes to the lower rpms by having the cylinder base cut. This moves the cylinder down, advances the port timing and raises compression, both of which increase bottom rpm power. Eric Gorr did this mod for $50 in the past.

In addition to the other mods mentioned by others, I would also advance the ignition timing a few degrees to increase bottom end power and reduce top end power.

Z....good point on the timing....forgot all about the...my EC 200 has a little timing.....

These are all great suggestions. We are going to start with a FWW, and probably a needle change. Maybe add some timing as well.