WE BUILD AN L.A. SLEEVE YAMAHA YZ270:

On Jul 7, 2010 Last updated Feb 15, 2018

If you own a 2006 to 2010 Yamaha YZ250, consider yourself lucky. You have one of the best motocross bikes ever made. It weighs less than a comparable four-stroke, costs less to repair and produces more horsepower per cc. And most importantly, it comes stock with Kayaba SSS suspension. It is a sweet motocross machine.

However, there is a but. If you are racing your YZ250 against 250cc four-strokes, you have a superior motocross machine; but if you race the 450 class on your YZ250, you are giving up a bundle of horsepower and torque to the oversized thumpers.

MXA wanted to take our trusty but rusty 2007 Yamaha YZ250 two-stroke and make it competitive for racing in the 450 class. Best of all, it was in desperate need of a top-end job and was coming apart anyway. We called L.A. Sleeve and ordered their YZ270 engine kit. It set us back $654.84.

WHAT DO YOU GET FOR THE MONEY?

The YZ270 engine kit includes five parts and two services. Here is the list:

Sleeve: The $120.00 L.A. Sleeve big-bore sleeve brings the 250cc bike up to 270cc. Labor to install the sleeve is $170.00.

Piston kit: We used a Wiseco high-performance, forged, dome-topped piston. It costs $142.98.

Wrist pin bearing: You may be able to recycle your old stuff, but why take the chance of running metal through the engine for a $16.00 kit?

Gasket kit: The $28.21 kit will bring you to L.A. Sleeve’s specs. If you want lower compression, ask about a thicker base gasket.

Head milling: L.A. Sleeve charges $100.00 to mill the head to fit with the larger piston.

Power valve mod: This $65.00 procedure is necessary for clearance of the piston.

Jet kit: This $12.00 kit includes three mainjets (two richer and one leaner) and two pilot jets. For our low altitude and relatively warm temperatures, L.A. Sleeve recommended going to the next largest mainjet (180), dropping the needle one clip position and going to a smaller pilot (50).

Externally the L.A. Sleeve YZ270 big-bore kit is not visible to the naked eye. Instead of replating the cylinder, L.A. Sleeve makes a steel liner that is pressed into the cylinder and then outfitted with an oversize piston. The sleeve allows the engine to be bored out several times…should the need arise.

HOW MUCH WORK WAS INVOLVED?

The cylinder and head need to be sent to L.A. Sleeve for the sleeve and head milling. Once you have all the parts, it’s a direct, bolt-on top-end replacement. Thank goodness for the simplicity of two-strokes. The only minor inconvenience is changing the jetting, which not all big-bore kits call for. This extra effort was worth it when we experienced how crisply the bike ran.

As a side note, L.A. Sleeve has a partially developed 330cc kit (that will be available soon). Its installation would be facilitated by having a brand-new cylinder with all the necessary components included.

WERE THERE ANY SPECIAL NEEDS?

Aside from the minor tweak to the jetting, the L.A. Sleeve YZ270 kit was very straightforward. That is, once we resolved a personal problem. Our particular 2007 YZ250 had sat with old gas in the carburetor for too long, and the needle-and-seat developed the nasty habit of sticking. After disassembling, cleaning and setting the float height properly (5.5mm-7.5mm) it was as good as new.

WHAT WAS IT LIKE TO RIDE?

The stock YZ250 powerplant has been amazingly usable for the last decade. On a ring-ding, a little low-to-mid power goes a long way toward being able to carry speed through low-traction sections and on the exit of turns. The L.A. Sleeve YZ270 is all about roll-on power. It boasts a healthy increase in the amount of torque and low-to-mid power?way beyond what the stocker offers. Best of all, it does so without sacrificing the desirable two-stroke snap. The YZ270 comes on the pipe with a little bit more oomph and then continues to rev out just as freely as the stocker. The meaty bottom-end let MXA test riders get onto the pipe with little use of the clutch, resulting in less wheel spin and more drive. The engine kit doesn’t add much, if any, power on top, but it doesn’t sacrifice any either, thanks to L.A. Sleeve’s cautious increase in compression.

MXA‘s goal was not to build a mini 450cc motocross bike, but to add some grunt and torque to the YZ250 two-stroke. We wanted to keep the character of the YZ250 but give it a fighting chance to get off the line, out of a corner or up a hill with the bigger 450 four-strokes.

After riding the bike, the first performance mod we tried was a 49-tooth rear sprocket to replace the stock 50-tooth. The YZ270 has more than enough bottom to pull one tooth taller at least. With the smaller rear sprocket, you can still go into most turns in the same gear as you would with the lower gearing, but you carry the gear longer and shift less often.

On the track the extra 20cc paid a big dividend in usability. The extra horsepower and torque were enough to make the Yamaha YZ250 two-stroke more competitive against its much larger 450cc competition. Amazingly, the powerband didn’t lose any of its flexibility and could be ridden like a snappy 250 two-stroke or torqued like a big-bore engine.

Ultimately, the YZ270 kit narrows the gap between the 250 and the 450 as far as torque and tractable power without losing the two-stroke’s character. It still gives up some power at peak, but for many it’s a small price to pay for a competitive package that’s light, agile and responsive.

For more information, visit www.lasleeve.com.