Is a 144cc Big Bore kit on a 125 worth it?

Looking for that little extra power to get you over a certain jump? Or is it because you want to show your four-stroke friends that two-stroke dirt bikes still have what it takes? Either way, a big bore kit can help you do just that.

YZ125s are the most common 125 in the US and for good reason. Yamaha is the only Japanese manufacturer that imports two-stroke engines into the United States. They are fast, light, reliable and fun. But, for some of us who just want a little more out of a small-bore two-stroke, an improvement in displacement is probably at the top of the mod list.

Just because it’s 144 cc doesn’t mean it’s 144…

You can get a 144cc bore cylinder, but in order for it to truly perform like a 144 it will need proper tuning. That’s why you’ll hear positive and negative feedback from people who upgrade their 125 to a 144. More often than not, it’s the people who buy just a cylinder kit that give bad feedback. This is because the cylinder, carb and exhaust are not tuned correctly for the extra cc’s. When I say a cylinder kit, I mean one that you can buy from a company like Athena and just bolt on.

You can’t go faster without more gas

Too many people think they can make their dirt bike faster by bolting on parts and doing nothing else. Motocross bikes are high-tech racing machines, so it is essential that they are well tuned. If you don’t adjust a bike after putting a “jump” part on it, it will probably run worse and possibly break shortly on the trail. More power requires more gasoline, so if the carb doesn’t feed enough fuel into the cylinder, it won’t work as it should. If your bike was cast correctly before you modded it, then it will run poorly afterwards. You will have to get on the main jet at the very least. A change in the position of the pilot jet and clip on the needle may also be necessary for optimal performance.

A bigger engine needs a bigger pipe

Take a look at the size and shape differences of a 125 to 250 two-stroke pipe if you haven’t before. Bigger engines need bigger pipes to take advantage of the increased displacement. Yeah, that means if you’re using stock pipe on a 144 then you’re probably not getting the most out of the motor. Some companies make special tubes for large bores, but they can cost a bit more (still cheaper than four-stroke exhausts). If you want to stick with FMF or Pro Circuit, research which tube works well for your dirt bike, because not all tube and bike combinations will give you the same result.

Build the Ultimate 250F Killer

Now if you bore your 125 two-stroke and have it built by a reputable bike manufacturer, it can be out of this world. In addition to boring and plating the cylinder, most builders will tailor the intake and exhaust ports to their liking (either low to midrange or mid to full power for most builders), change the port timing They possibly do some modifications to the casing. If you ship your entire engine and cylinder head mods if you want higher compression (will require race gas). As a result, this will create an incredibly fast small-bore two-stroke that will outperform the 250F and keep pace with the 450 with an experienced rider. Oh yeah, did I mention that most stores can do this for a relatively low price as well? That’s right, who needs a big-ticket four-stroke when you can eat it up with a well-tuned 125/144?

Athena didn’t know what they were doing…

While I’ll admit I’ve never owned a 125 with a 144cc Athena kit, I’ve done a lot of research to find out what’s up. In fact, I didn’t have to look very far to find out if it was worth it or not. Review after review showed that the top-end Athena kit did not improve the stock YZ125 much. They basically took the stock cylinder, increased the size and power valve ports, and called it good… You can see a slight increase on the dyno, but to really feel the effect of the extra cc you have for use it as you would a standard cylinder. Port sizes, shapes and lengths will not be the same as a 125cc if fitted correctly. This is where the reputable two-stroke engine manufacturer comes into play. There are builders out there who really know how to open a 144 up to its potential, so don’t settle for less.

Here is a great example of an Athena YZ144 kit test done by Motocross Action magazine. They do an in-depth overhaul, and in the end, their YZ barely makes two extra ponies, and that’s with a GYTR pipe and muffler made for the 144! If tuned right I honestly think it should be able to make 5-7 more hp than a stock 125.

-Tom Stark