01 YZ125 set up

[fastbygaff]

Just purchased this bike for my son. We haven't really tested her out yet due to weather but on the surface she looks graet.

My test ride of her found the front brake lever needs replaced, clutch some what grabby and out of adjustment (most likely needs oil change). One thing I didn't like was a lack of bottom end.

Only mod I see so far is a new SST pipe with a shorty. It's most deffinate that the top end needs done. No other reason other than when I asked the last time the top end was done I got a stubid look and a answer of "we were thinking of doing that".

So before I jump in, whats the set up for low end MX set up, and whats the pro's/con's of this one.

My comment about low end power may be wrong due to I ride the KX-250 with close to no time on a 125 in about eighteen years.

 

[Detonator]

Fastbygaff,

My last bike was an '01 YZ 125 that I used for woods racing, set up for low to mid rpm riding. Before you get a bunch of people who jump on here and blame the big 38mm carb for "low end problems," I'm going to beat them to the punch by saying this bike has one of the most usable motors I've ridden, and coupled with the 6 speed tranny, it is a versatile machine. Mine was ultra-reliable with frequent oil and air filter changes. My exhaust was left stock to keep it quiet, and I repacked the silencer twice a year. You might want to hold off on the pipe change until you've tried some less expensive alternatives.

First, if you suspect you need a new top end, that may be the source of your low-end power issue. At the same time, you can take the opportunity to clean your carb and powervalve (a sticky or gooped up powervalve will kill your low end power.) Another $5 on silencer packing is a good investment if the previous owner spooged it up.

Second, this bike needs to be jetted correctly, but is not "fussy" regarding elevation or temperature once you get it there (within reason). I would highly recommend some Boyesen Power Reeds to give you crisper throttle response and a low end with a bit more bark. Your stock ones are probably tired by now. You'll have to change your main jet from a 460 to a 420, but the other carb settings remain stock. Scroll down and read here: http://www.boyesen.com/article/articleview/60/1/4/ I got best results with a plug one range hotter than stock, but I live in the Great White North, so you might just want a fresh stock spec plug.

Third, the most cost-effective cure for a lazy low end on a 125 is to either go a couple of teeth more on the rear sprocket, or for a radical change, one tooth less on the front sprocket. I ran one tooth less on the front sprocket and threw on a quality O-ring chain. I needed to adjust it perhaps twice over the last year, and enjoyed the reduced chain maintenance. I think there may be an issue with the rear chain guide if you're trying to fit an O-ring if you go bigger on the rear sprocket...make some calls before you go that route.

Fourth, now that your gearing is lower, your next issue is wheel-spin. This bike has a motor. I ran a 9 oz Steahly flywheel which took nothing away from the engine's willingness to rev, but put the power down in a more tractable, usable way. It's an understatement to say the bike responds well to the flywheel weight. You might want a slightly lighter weight for MX versus my offroad use, but this was the one mod I couldn't live without for this particular bike.

Fifth, and most obvious, is to make sure your suspension is helping to put power to the ground and working with the motor. After my revalve I really enjoyed the increased traction, willingness to pull wheelies, and ability to power through smaller chop. It allowed the motor to do its job instead of fighting one another.

This may sound like some work, but you just bought a used 125, and they always require some love and cash to perform at peak. Just to save you misery, mine hated Yamalube in the gearbox. Oils are like religion around here, but my particular faith was Golden Spectro Clutch Saver in the box and Golden Spectro SX Synthetic in the gas.

 

[va_yzrider]

I agree with everything Detonator said. I had an '01 YZ 125 and it flat out ripped. I miss that bike so much. The key is to get the jetting just right. I also went up to a 51 tooth rear sprocket. As far as exhaust, I had a Pro Circuit Platinum and PC shorty silencer. The SST is known to shift power to the mid to upper range, so that could be an issue. If the bike is ridden in the power band and on an MX track, the SST would be a good choice. Also, as Detonator said, make sure the powervalve is clean. Do this when you do the top end. I also ran a 50/50 mix of 109.5 race fuel with 93 octane to keep it a little cooler and supplement my jetting. I mixed the fuel at 36:1 with Bel Ray H1R and it was spot on. I stayed away from the O ring chain on this bike, but it may not be that big of a deal. The heavier chain seems to rob a little bit of power, but it is a trade-off with maintenance. I run an X-Ring DID chain on my YZ 250 and am completely happy with it.

All said, the '01 YZ 125 is a sick bike that will perform extremely well if it is set up properly. Spend time on the jetting and you won't regret it.