144 or 250

Reliability is going to be crucial of you're thinking about going 144. I believe there are VERY FEW tuners that can build a 144 that will be as reliable as your 125 so proceed with caution.

I had 2 KTM SX 150s before I got the KTM SX250 2-stroke I have now. After riding it for a month you will say it is just as nimble as a 125. In fact, a 250 feels a lot more 'stable' without feel heavier or less nimble. Depending on your age, weight and experience, chances are you'll have more fun on one too because a 250 actually has some bottom end, lol.

In the US you should be able to pick up a YZ250 from 06-14 without breaking the bank. The SX250 is a better bike in my opinion, but you would have to get at least a '12 model to have a bike from the latest generation, and it will have a lot less value for your money than the YZ.

Hope that helps.

Do the 144 kit the right way and you will not regret it! You can get 40-42hp out of them fairly easily and it will still handle like a 125.

Who does the 144 kit the right way ?

Here is my experience...I had a 2000 rm125 that I wanted some more power out of. I did the v force, exhaust, etc. Then I decided to sent the motor to Eric Gorr to do the 144 conversion. I had to send it to him because unlike the yz, no one makes a 144 kit (Athena makes one for the yz). The power was awesome and it still handled like a 125. I never had any durability issues with it. But I ended up selling it and buying a 09 KTM 250sx. Now that bike had plenty of power, actually too much for me. The only problem with it was the WP suspension. I had it revalved by a factory KTM suspension guy but I just never could get used to the suspension. It always felt harsh. But other than that, the KTM was an awesome bike. It felt wide compared to the RM and while it turned well, it definitely didn't turn as well as the RM. If your happy with your YZ, then look into the 144 kit. If you really feel like you need more power then buy a used YZ250 or new KTM (like Fearo said).

In retrospect, I really wish I would have kept my RM

You can buy an Athena kit and bolt it on for a 3hp gain, or send your stock cylinder to a good tuner and make some real power.

With my Husky I had the head cut for 8.5mm squish and a thinner 0.4mm base gasket to bring the compression up to around 190psi. The exhaust port was polished, transfer channels were knife edged, and rear transfer port raised to the stock 125 height, also deepened and widened. Also had the powervalves reground for a tigher seal, which really boosted the bottom end without sacraficing anything on top. Combined with a Scalvini pipe it made stupid power. Dyno numbers don't mean much when there is no baseline but it shocked the guys at the shop when it did multiple 42hp / 21ft-lb runs. This with just some minor tweaking, and still using the stock Mikuni carb.

If I'd kept it as a 144 I would have sent the whole engine and had the crank balanced, gone to a domed piston for higher compression, played with ignition timing, had the cases matched, and experimented some more with deck height. But it ran so good already I just wanted to ride it and not wrench on it. It was surprising how easy it was to get big power out of it, I can't see why the YZ isn't capable of the same.

It's a 165 right now, my next project will be turning it into a stroker 177.

V force Rad valve ?

Eric Gorr did the work on my 94' CR144. A properly built 144 is every bit as reliable as a 125.

I can vouch for EG he use to build my race motors and big bores (310 kxs.lol) before i got some brains and got away from the 250s. If you want to stop having fun riding get a 250 or even better a 450f if not go BB.