LAPCrightwebHorsepower is the most expensive addiction. You can spend a fortune chasing it, and the returns inevitably diminish.
With the LAPC YZ250F, we were determined to know when to say no. The 2015 Yamaha YZ250F was already a great bike. It won our 250 MX shootout and had no glaring weaknesses. As always, though, peak power is disproportionately important in the 250 class, and it wouldn’t hurt to find a couple more ponies, especially on top. We looked to the guys at L.A. Sleeve to see what we could do without breaking the bank. We weren’t interested in changing the displacement, but we knew that they had other ways of finding cheap power.
For years Yamaha championed the five-valve head. The moment in time when five valves made sense has passed, however, and starting with 2014, the YZ250F has a more conventional four-valve design. That’s good news for engine modifiers, because there are 20 percent fewer valves to replace, modify and pay for. Like most engine builders, L.A. Sleeve is a big proponent of the five-angle radius valve job, which is too labor-intensive for production but results in a much better seal. The stock valves can be retained, and the whole job is $200.
The head itself has some hidden power. Again, this is simpler and more cost-effective than it was in the five-valve days. A CNC-machined head from LA. Sleeve costs about $300. L.A. Piston Company is L.A. Sleeve’s in-house piston company, and they say their offering for the YZ has lighter clearance, less noise and better ring seal than stock. The price is $250. From there it was just a matter of giving the YZ a new exhaust system. The DR.D stainless/aluminum full system sells for $599 and is no louder than stock. We also installed the DR.D radiator lowering kit, which sells for $119.95.
RON_5563webAfter racing the stocker for a few months, the clutch began to slip. We more or less expected this, simply because the clutch pull is so easy. The stock springs are borderline too light with the motor in stock condition, and any increase in horsepower would require a spring kit. We decided to go a little overboard and install a complete Hinson clutch assembly: springs, plates, hubs and pressure plate. This cost us over a grand in clutch parts, but it will actually pay for itself over the life of the bike. At the very least, count on installing stiffer springs.
For suspension, again, the Yamaha needs very little attention. We lean toward off-road events, so we wanted our YZ to work for the Big Six GP series. Precision Concepts specializes in that kind of setup. As usual, the price for suspension modification is highly variable, depending on the rider’s skill level, needs and the condition of the original parts.
We also got to try an interesting front-brake setup. The stock Yamaha probably has the weakest brakes in its class. Part of the reason is that it has smaller brake pads with less surface area. Even though the brakes seem to be the same as Kawasaki’s and Honda’s, they aren’t. MotoStuff.com offers an oversized brake kit that allows the use of Kawasaki pads. The kit includes a new carrier, a Braking rotor and the pads.
RON_5559webRIDE IT!
The LAPC YZ250F came together about when we finished building the racecourse for the John Burr Six Hours of Glen Helen. That’s the fringe benefit of working with Glen Helen Raceway to set up their endurance race series: we get to do as many laps as we want before the race starts. Our YZ250 was the perfect test subject for the 9.5-mile GP course. It was part motocross, part singletrack and part fire road. The bike couldn’t have performed better. On the motocross course, it was in its element. The power increase was just substantial enough to allow the YZ to pull the big Glen Helen hills without suffering the way a stock 250 would. And, we already knew it would work well on the singletrack. The Yamaha is lighter than any other 250 four-stroke and could positively eat the 450s alive in tight trails.
If we were actually going to race the Six-Hour or a similar GP, we might have to gear up. The stock gearing tops out pretty quickly. The new powerband is also very top-end-oriented. It makes its power peak just before the rev limiter in the ignition shuts down. That means you’re also riding the thin line of trying to get the most out of the motor and shifting at the right time. It isn’t that the motor makes any less power down low; it’s just that the added power surge on top is so fun that you find yourself up in the high ranges most of the time. Gearing would help that as well.
We’re delighted with the results of the project. We spent far less than we do on most projects and gained a noticeable improvement. Yes, there’s always the urge to do more. But, in this case, we pushed away from the table at the right time. For more info on the parts that went into this build, click here.


Comments are closed.