Is A 144cc Big Bore Kit Worth It On A 125?

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

YZ125’s are the most common 125’s in U.S., and for a good reason. Yamaha is the only Japanese manufacturer that imports two-strokes to America. They are fast, light, reliable, and fun. But, for some of us that just want a little more out of a small-bore two-stroke, an upgrade in displacement is probably on top of the list of modifications.

Just Because It’s 144cc, Doesn’t Mean It’s A 144…

You can get a bored out cylinder that’s 144cc’s, but to actually perform like a 144 it will need some proper tuning. This is why you will hear positive and negative comments from people who modify their 125 to a 144. Most of the time, it’s the people who buy just a cylinder kit that give bad feedback. This is because the cylinder, carburetor, and exhaust are not properly tuned for the extra cc’s. When I say a cylinder kit, I’m talking about one you can buy from a company like Athena and just bolt it 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 not do anything else. Motocross bikes are high-tech racing machines, so it’s critical that they are finely tuned. If you don’t tune a bike after putting on a “hop-up” part, it’s probably going to run worse, and possibly break shortly down the road. More power requires more gas, so if the carb isn’t feeding enough fuel to the cylinder, it’s not going to run like it should. If your bike was properly jetted before you modified it, then it will run lean after. You will have to go up on the main jet at the minimum. A change in pilot jet and clip position on the needle may also be required for optimum performance.

Bigger Engine Needs Bigger Pipe

Take a look at the size and shape differences from a 125 to 250 two-stroke pipe if you haven’t before. Bigger engines need bigger pipes to take advantage of the higher displacement. Yes, that means if you’re using a stock pipe on a 144, then you probably aren’t making the most out of the engine. Some companies make special pipes for big-bores, but may cost a little more (still cheaper than four-stroke exhaust by a long shot). If you want to stick with FMF or Pro Circuit, do some research on which pipe works well for your dirt bike, because not every pipe and bike combination will give you the same result.

Build The Ultimate 250F Killer

Now if you get your 125 two-stroke bored and built by a reputable bike builder, it can be out of this world. In addition to boring and plating the cylinder, most builders will port the intake and exhaust ports to your liking (either low to mid-range or mid to top-end power for most builders), change the port timing, possibly do some case mods if you send your entire engine in, and cylinder head mods if you want higher compression (will require race gas). In result, this will make a screaming-fast small-bore two-stroke that will outrun 250F’s and keep up with 450’s with an experienced rider. Oh yeah, did I mention most shops can do this for relatively cheap too!? That’s right, who needs a high-buck four-stroke when you can eat them up with a finely-tuned 125/144…

Athena Didn’t Know What They Were Doing…

While I admit that I’ve never owned a 125 with an Athena 144cc kit, I’ve done plenty of research to find up what’s up. In fact, I didn’t have to go very far to find out if it’s worth it or not. Review after review showed that the Athena top-end kit did not improve the stock YZ125 by much. They basically took the stock cylinder, increased the power-valve size and ports, and called it good…… You can see a slight increase on the dyno, but to really feel the effect of the extra cubic-centimeters you have to port it like you would a stock cylinder. The port sizes, shapes, and durations will not be the same as a 125cc if tuned correctly. This is when the reputable two-stroke engine builder comes in. There are builders that really know how to open a 144 up to its potential, so don’t settle for less.

Here’s a great example of a YZ144 Athena Kit Test done by Motocross Action Magazine. They do an in-depth review of it, and in the end their YZ barely makes an extra two ponies, and that’s with a GYTR pipe and silencer made for the 144!! If it’s properly tuned, I honestly think you should be able to crank out 5-7hp more than a stock 125.

-Tom Stark