APPLICATION: Most late-model mxers

PRICE: $149.95 (adjustable leak); $14.95 (accelerator pump spring); $22.95 (fuel mixture screw)

MOTOCROSS.COM RATING:9

CONTACT: www.mergeracing.com

            Modern day four-stroke carburetors are complex pieces of machinery. They do a lot for a motocross bike, and they also offer a lot of different aspects of adjustment for fine-tuning. One such way is the often over-looked leak jet, because despite what you might think, this integral part on the carburetor plays a big part in how a bike performs.

            The correct way to set up a modern carb is to make sure it’s jetted properly, then adjust the amount of fuel fed to the engine in order to properly match up with the airflow that is controlled by the jetting. When the throttle on a modern four-stroke is cracked open, fuel is shot straight into the cylinder by the accelerator pump. What the leak jet does is allow excess fuel back into the float bowl, and the smaller the leak jet, the greater the flow of fuel is into the engine. In other words, the leak jet plays a big part in controlling the amount of fuel that enters the cylinder. The problem with fine-tuning this part of the carburetor is that the jet is very difficult and time consuming to change. This is where the Merge Racing adjustable leak jet comes in.

            By being adjustable, you can properly set the carburetor up based on jetting or other variables such as heat and altitude. The Merge Racing leak jet makes it easy to fine-tune by simply turning a screw, which in the end should leave your bike performing at its best, eliminating bog and hesitation. Our carb was set up with not only the Merge Racing adjustable leak jet, but the company’s accelerator pump spring and fuel mixture screw as well. The accelerator pump spring is 50 percent stiffer than stock and is designed to improve throttle response, while the fuel mixture screw doesn’t offer really any performance gains other than it being brass rather than aluminum and you can easily get to it for adjustment.

            After tearing our KXF apart to swap out the carb—which took us a little while since we removed the shock, subframe, gas tank, and disconnected anything and everything from the stock carb to get it out—we took to the track to see how the set up performed. Initially, we didn’t notice a drastic change in the way the bike performed with the adjustable leak jet installed, however, once we settled in and got to pounding out a moto, we noticed a difference. The throttle response was improved with the accelerator pump spring installed, and bogging or hesitation was nearly non-existent due largely to the leak jet. Usually, when you ride a bike for an extended period of time, which we did in this test, bogging and hesitation can increase, however, with these mods installed, we never experienced this. One thing that we don’t like about the adjustable leak jet, though, is that installing it is a pretty big chore. The instructions require you to drill a small hole in the float bowl, so installing this new adjustable jet is far more permanent than some people might like. For us, though, the new mod and improvements offered make it a worthy investment, and if you want to return to stock, it really isn’t that difficult—all it takes is purchasing a new float bowl or plugging the small hole.

RATING

            Like we mentioned, the Merge Racing adjustable leak jet and carburetor mods do make a difference, in that they nearly eliminate bog and hesitation of a stock four-stroke carb, and also increase throttle response. But to us, installing this modification is quite a task, however, if you’re a serious racer or rider, performing a mod like this is a small price to pay for the performance gains and the ease of adjustability you get from the adjustable leak jet, accelerator pump spring, and fuel mixture screw from Merge Racing.