7 Best YZ125 Mods That Make A Difference

Want to make your YZ125 faster? Whether you’re looking to build the fastest 125 2 stroke or just want a powerful and light trail bike, this guide will cover the top 7 mods you can do that will actually make a difference. 

2003 YZ125 W/ 134cc big bore

I’ve owned several YZ125’s over the years, and there’s a lot of upgrades available for this motocross bike, but they don’t all have the same effect when it comes to an actual improvement.

The best performance mods and upgrades for your YZ125 are:

  • Jetting
  • Suspension tuning
  • Big Bore
  • Porting
  • Reeds
  • Pipe
  • Cylinder Head

Best Bang For Your Buck Mod: Jetting

The single best mod for most 2 strokes, including the YZ125, is getting the jetting right. You can easily gain torque, horsepower and throttle response in minutes, even if your bike seems to be running well. 

This 2 stroke jetting guide will show you how it’s done. 

Suspension Tuning

Once you get your YZ 125 running strong, the next best thing to do is get the jetting dialed in. 

The spring rates need to be correct for your weight and type of riding. See this spring calculator for an estimated number. Stock dirt bikes are usually set-up for 160-180lb riders. 

If you have the correct springs in, you need to get the bike balanced to handle properly. This is done by setting the ride height. 

Adjusting the shock sag is the first thing you should do to set the ride height. The forks can be adjusted by moving them up or down in the triple clamps, as well as adjusting the preload with an added spacer. 

Adjusting the compression and rebound clickers are the next step to tuning. Try turning the clicker in 5 clicks at a time until you notice a difference. Trial and error until you find the most comfortable setting for you. 

Big Bore Kit Worth It?

Going from 124cc to 144cc is a 16% increase in displacement. You can expect that much of a power gain with proper jetting. 

Slight case mods are required, but these can be done without splitting the cases if you’re careful and take necessary precautions. 

Going with the Athena 144 big bore kit for your YZ125 allows you to do everything yourself, which allows you to save a few bucks and you know what’s actually going on in your engine.

On the other hand, if you don’t have the time or confidence to do the engine work, you can send it out to a reputable builder, such as Eric Gorr or 

Porting

Cylinder porting is where you can make or break the power-band on a 2 stroke engine. The YZ125 has pretty good ports from the factory, but there’s always room for improvement. 

If you don’t know what you’re doing, I strong warn you NOT to port the cylinder yourself. It’s very easy to make it run worse. Going bigger with the ports is not always better. In fact, sometimes smaller in some spots is better for overall performance. 

Find someone that knows the YZ125 engine and can give you the type of power-curve you want. This could be a low-end boost in torque and throttle response for woods riding, shifting the power towards the top-end and over rev for a screamer, or somewhere in between. 

Reeds

Stock reeds do a good job, but they’re nothing special when it comes to performance. 

The new Moto Tassinari VForce 4 reeds have been developed to give you sharper throttle response and more over rev.

Their new design is more efficient, lasts longer, and provides more air which increases power. 

In the end, you can only gain so much from a Reed change, but the VForce4R will make a noticeable difference, and it costs less than a new pipe. 

It’s also fairly easy to install. If you can remove the carburetor from your YZ125, you’re halfway to changing the reed cage, which is just a few more small bolts. 

Pipe

The pipe, next to porting, is the biggest factor to determining the power curve. 

Two of the most popular pipes for a Yamaha YZ125 are the FMF Factory Fatty and the Pro Circuit Works pipe. 

Kris Keefer tested both of these with a respective shorty silencer from FMF and Pro Circuit. 

His results after riding the bike back to back with each exhaust system came back with the FMF setup being slightly more upper-rpm and over-rev biased, while the pro Circuit was a little better on the low-end and mostly mid-range. 

Based on those results you choose what you want for power characteristics. I would choose the FMF Factory Fatty and FMF shorty for big and fast motocross tracks, as well as open desert riding. 

The Pro Circuit Works Pipe would be better for a beginner or if you want to lug your bike more and short-shift. 

Cylinder Head

Increasing compression is one of the easiest ways to gain power. 

An aftermarket cylinder head will give you more torque and better throttle response, as well as make jetting easier. This is done with correcting the squish clearance between the top of the piston and the head. 

A stock YZ125 has a loose squish tolerance, which is simply due to Yamaha manufacturing it on the “safe” side. Too little clearance will require race gas or engine failure. 

The VHM and Phathead are the most popular aftermarket cylinder heads for 2 strokes. They both offer a variety of combustion chamber inserts for differing compression ratios. You can set it up to run on pump gas or race gas. 

Kelley Fager

I help new riders learn how to safely ride and understand how to tune and fix their dirt bike in their garage.

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