When it comes to 250cc 4-stroke motocrossers, the Lites-class bikes are intended to be ridden rapped out, bouncing off the rev-limiter. Due to the smaller displacement, aggressive riding is required to keep these 250s going without losing momentum. Any sort of power enhancing modification can be huge, and the easiest and way to unleash hidden ponies from your ride is to replace the stock exhaust with an aftermarket system.
We installed an FMF Factory 4.1 RCT MegaBomb exhaust system on our 2012 Yamaha YZ250F project bike.
Manufacturers produce bikes with exhausts designed to to meet sound regulations. Stock exhausts are built to be as quiet as possible, but but there are often extra power gains just waiting to be untapped. Aftermarket exhaust systems open up passage through the muffler as well as header pipe, the pipe specifically shaped for optimum power out of your bike.
We took our 2012 Yamaha YZ250F Project Bike over to FMF Racing for installation of a FMF Factory 4.1 RCT full exhaust system
. We then got some final motos in before returning our test bike to Yamaha. FMF replaced our stock exhaust with their signature MegaBomb header and the Factory 4.1 muffler. Fit was about as precise as it gets. The Megabomb is clear of any hoses around the front of the engine. The muffler easily slipped right onto the header with no hesitation and lined up perfectly with the frame bolt holes.
Before riding, we scoped out the visual aspects of the FMF Factory 4.1 exhaust system. “Factory” is in the name because that’s exactly what it is – a factory pipe. Purchasers of the system receive the same exhaust ran by riders like Andrew Short and Davi Millsaps. A titanium header and mid-pipe with some of the cleanest welds known to man along with a titanium muffler canister and carbon fiber end cap, it doesn’t get much more factory than that.
Because the Yamaha is a carbureted bike, changing the jetting was optimum in getting the YZ to run correctly with the new exhaust. FMF kindly dialed us in, so it was time to ride. After heading out to the track for the first time it was noticeable after just a couple laps, the jetting just wasn’t right. The bike was back-firing going into corners after letting off the throttle and getting a gnarly bog on corner exit.
We had our boys at FMF give the jetting another go and it was off to the track again. For the second go around, we headed out to our favorite, Zaca Station Motocross Park. We notice at idle the bike seems to run good when warm. On the track when
Looking at the dyno charts, the power curve begins to overtake the stock results at 8,600 rpm and builds over a 1.6 horsepower advantage.
compared to stock, the FMF broadens bottom end power as we are able to sense in slower tighter corners. The bike blasted out of corners with much more usable power grabbing traction, with the help of our Bridgestone M403 front
and M204 rear
tires, instead of revving to the moon and going nowhere.
At higher speeds the 250 is a different story. Though improved on the bottom end, the jetting still needed fine tuning to run at prime up top. When taking on a load through roller sections or jump faces the engine constantly cut out, triggering us to come up short on jumps and drop the front end in rollers and whoops.
On the dyno, the power curve ramps up much more linear than stock and gains just over a 1.6 horsepower advantage (35.61 vs. 33.99) . While power with the stock exhaust levels out at 8500 rpm, the FMF curve is still gaining power until it drops off at 12,000 rpm.
Although we would like to fine tune this issue our time with the YZ-F is up as 2013 is coming in hot. However, we plan to keep the FMF Factory 4.1 exhaust for our 2013 bike since Yamaha opted to stick with the carburetion instead of fuel injection. So the old school way of tuning power is optimized, and we can keep dialing in the jetting.