2004 YZ125 Single Track...Need more bottom end power!

Unfortunately 125s aren’t known for their bottom end and you would have to throw a lot of money at it to get some I. E. pipe, silencer. A 7-10oz flywheel weight ($100) would make the power smoother and more tractable. Your best bet is to improve clutch skills to keep it on the pipe. 125s and 80s have smoked my 450 on tight tracks and trail’s because they are better riders. You will get there too. Just keep at it. 

Corner speed. I race my 125 off-road and that’s your best “mod” for making you and the bike work less. It’s really hard to get good bottom end out of a motor that was made to be revved past 10k. 

I second the flywheel weight. It will help keep you from stalling. 

I've ridden at the White Tank mountains... sandy with small rocks on top? Yeah, that's hard on a 125. Maybe try a 52-tooth rear and an FMF Gnarly pipe, if they make one. 

Fly wheel weight and punch it out to a 150 or 167 on the next rebuild

Jetting! Get that burn good in the 1/4 to 3/4 throttle position. Nice lower end pipe, then a flywheel weight, but not too much. Just my opinion. 

There isn't a Gnarly pipe for the 125, so your fatty will have to do. The JD jet kit might be worthwhile.

If you are going to just ride trails with it and aren't too worried about screaming to end, then have the cylinder base cut to move the power down in the rpm range a bit and the head cut to go with it. 

For the price the Tusk recon is a pretty good tire for hard terrain. 

I would contact them, and ask what they recommend. I'm sure the jetting will help as well as an appropriate flywheel weight.  As you said, a 7oz would probably be ballpark for a 125. 

Best mod you can do on a 125 but you’re not going to like it. Lose as much weight as possible in the gym. 

The tire runs big, so that may not be a good option. I just looked and they don't make a 100 so it may not be a good choice. I didn't realize that when I suggested it. 

An AT81 or VE33 might be a better choice. You can probably get away with a 110 in either of those.  

Here are a few tips and mods that you can do for more low-mid.  It is a YZ125, so there are only a few things that can really be done to tune it for that, it still needs to be ridden harder.

Stroke it, overbore it, or overbore and stroke it.

A light FFW and all steel clutch plates.

Retard the port timing by shaving off material on the cyl base, keep final squish clearance to 1mm...-.015" to .020" will help but it will take away a little top end overrev.

Epoxy and profiled rear transfers so the ports appose each other and short circuit the fuel at each other rather than face the exh port.."remove the hook" in the rear transfers.

A pipe that adds mid-range

Dual stage reeds

Slightly smaller carb

Good luck-

A flywheel weight works nice on the trails , also don't forget cutting the head for more compression will boost the bottom end a good bit. 

Increasing the compression by just a head cut will mainly “excite” the current powerband and characteristics over just increasing the low end power…with just a head mod it will add a more abrupt power delivery.

lowering the cyl base to increase the effective stroke, (lowering the exh port in relation to the crank pin), will yield more low/mid torq.

Its nearly a 20 year old 125, being used for application outside its design.  Just keep it well maintained and ride it and have fun.

So many of the modifications described here will either cost more than the bike is worth or create setup issues that will drain your fun.

Man Bruce,

Im sure he's aware its not the best tool for the job.

Dont kill a mans good time, doing mods and enjoying the upgrades will add to the fun in my book not drain. 

Good luck then, taking a 20 year old bike and doing big bores and strokers just to go trail riding is a recipe for misery, not fun.  Messing around with the carb to get all these mods running right will suck, right before you push the bike 2 miles back down the trail when it seizes up.

Been there and done it, lol. Maybe a 133 kit with porting is possible since the 144 will need cases mods and the pistons wear very quickly. 

As for bolt ons,  most of the time they don't work as stock. 

Maybe, just maybe the OP could find a 36mm carb and manifold from a 1999-2000 I think would make it run cleaner down low.

Honestly, at your size and weight, sell it and get a yz250. Its very difficult to get a YZ125 to become a "good" trail bike. It likes to be screamed. Good trail bikes like to be lugged. I have a YZ250 and its an awesome trail bike with easy modifications. 

If your going to keep it, look at Apex Head for it. Completely changed the power of my bike. Best add-on I did to it. 

https://atinno.com/yamaha

https://www.rb-designs.com/rb_designs_llc_011.htm

Raced my 05 YZ125 in the AMA West Harescramble & D36 Cross Country as a B Senior and Super Senionr for many years with the RB Design mod carb. Google it... Theres plenty of favorable reviews. 

It's not stump pulling lowend but seat of the pant difference over a stock. I ran OEM and  a FMF Factory Fatty with a Leo Vince spark arrestor, 13/50 gearing, 3.2 IMS tank and Utah Sportcycle bash plate. Applied clamps, Fastway /GPR stablizer with Fastway hand guards. For enduro I used a ICO Checkmate with remote thumb switch. Super easy bike to ride in the woods / single track. Yes... you gotta be aggressive riding style, but the fun factor is off the charts. 

Ruy Barbosa > https://youtu.be/jaSq6xurWZ4

MX33 or similar. The sand tire would probably lose chunks when you hit the harder areas and those quartz ledges.

What you running before? Sparky often do that, they are long and have a lot of metal so the oil could condense, especially with extra restriction.

If you are trail riding, maybe adjust air screw or one smaller pilot and maybe 1 clip leaner pilot. 

Yeah sparky will clog her up.  And UNi's tend to be kinda thick with the 2 layers and all the oil and will richen the bike up a little too.  You'll need to lean out the jetting accordingly as Bruce said.  For tighter, single track type off road you generally want the bike leaner anyway since you're not doing much hard, wide open throttle and dont want the bike loading up and fouling plugs in the tight stuff.  Having it nice and crisp will also help with that low end response you are looking for.

Going to 40:1 (less oil) will richen the jetting even more.  Since there is less oil, that means there is more gas, per volume.  That will cool combustion temps and cause more spooge.  So, if you go to 40:1 you will need to lean out the jetting even more.   I'd stay at 32:1 for now and lean the jetting out.   One thing at a time so you don't get lost in the woods.

Also it works be a good idea to replace the midblock gasket if it's never been done.   Jdjetting sells a good kit for the mikuni and the replacement oring for the keihin is available (I don't remember where though).