Are you looking for that little extra power to overcome a certain jump? Or do 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 with that.
YZ125s are the most widely sold 125s in the US, and with good reason. Yamaha is the only Japanese manufacturer to import two-strokes to America. They are fast, light, reliable and fun. But for some of us looking to get just that little bit more out of a small-bore two-stroke, a displacement upgrade is probably high on the list of modifications.
Just because it’s 144cc doesn’t mean it’s a 144…
You can get a rebored 144cc cylinder, but to actually perform like a 144 it needs to be properly tuned. Because of this, you’ll hear positive and negative comments from people converting their 125s into 144s. Most of the time it’s the people who just buy a cylinder kit that give bad feedback. This is because the cylinder, carburetor and exhaust are not properly matched to the extra cc. When I say a cylinder kit, I’m talking about one that you can buy from a company like Athena and just bolt on.
You can’t go any faster without more fuel
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’s important that they are fine-tuned. If you don’t tune a bike after installing a “hop-up” part, it will likely run worse and possibly break shortly thereafter. More power requires more fuel. So if the carburetor isn’t feeding enough fuel to the cylinder, it won’t run as it should. If your bike was properly hosed out before the rebuild, it will run lean afterwards. You must at least ascend with the main jet. A change in pilot jet and clip position on the needle may also be required for optimal performance.
Bigger motor needs bigger pipe
If you haven’t already, take a look at the size and shape differences between a 125 and a 250 two-stroke pipe. Larger engines require larger pipes to take advantage of the higher displacement. Yes, that means if you’re using a stock pipe on a 144 you’re probably not getting the best out of the engine. Some companies make special big bore pipes, but might cost a bit more (still cheaper than four-stroke exhaust by a mile). If you want to stick with FMF or Pro Circuit, do some research on which tube works well for your dirt bike, as not every tube and bike combination will give the same result.
Build the ultimate 250F killer
Now if you bore your 125 two-stroke and have it built by a reputable bike builder, it can’t be 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 power or mid-to-high power for most builders), change the port timing, and possibly make some case modifications when customizing yours send in entire engine, and cylinder head modifications if you want higher compression (requires race gas). The result will be a screamingly fast, small-bore two-stroke that will outrun 250Fs and keep up with an experienced 450 rider. Oh yeah, did I mention that most stores can do this for relatively cheap too!? That’s right, who needs a high buck 4-stroke when you can eat it up with a finely tuned 125/144…
Athena didn’t know what they were doing…
While I’ll admit I’ve never owned a 125 with an Athena 144cc kit, I did a lot of research to find out what’s going on. In fact, I didn’t have to go very far to find out if it was worth it or not. Review after review showed that the Athena top-end kit did not significantly improve the stock YZ125. They basically took the stock cylinder, upsized the power valve 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 inches you have around it like one porting the stock cylinder. The port sizes, shapes and durations are not the same as a 125 when properly adjusted. This is where the reputable manufacturer of two-stroke engines comes into play. There are manufacturers who really know how to open a 144 to its potential, so don’t settle for less.
Here is a great example of a YZ144 Athena Kit Test conducted by Motocross Action Magazine. They do a thorough review 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 adjusted properly I honestly think you should be able to crank out 5-7 more horsepower than a stock 125.
-Tom Stark