Don't you just love jetting your carb?
Ok, probably not. In fact, jetting your dirt bike is probably one of the most, if not the most dreaded part of maintenance. Unfortunately, it's not something to deal with later and keep putting off. Racers especially need to stay on top of the fuel/air ratio entering their dirt bike's carburetor to ensure optimum performance.
In simple terms jetting your dirt bike's carburetor is determining the proper air-to-fuel ratio the engine receives. Running the correct ratio of fuel and air makes your dirt bike run optimally. Installing jets of different sizes helps increase/decrease the fuel/air ration depending on what your bike requires.
Most dirt bikes off the factory floor might look pristine but probably need jetting. A number of factors influence whether or not you'll need to jet the carb including the following:
- Upgrades to engine or exhaust system
- Basic routine maintenance
Four strokes tend to be more forgiving then 2-strokes when considering the factors above however riding at sea level (where bikes run leaner) and then in the mountains (richer) messes with any bike's jetting.
Is My Dirt Bike Running Rich or Lean?
After breaking in your dirt bike it's easy to determine whether you need to jet the carb simply by looking at the spark plugs. Take the plugs out and if they look clean you're good to go. However, if you see black soot then you're running rich - meaning you've got too much fuel - if you see white residue then you've got too much air flowing in or you're running lean.
You can also determine if you need to jet the carb based on how the bike rides. If your dirt bike tends to slough when you punch it and/or backfires when you open it up a bit, you're probably injecting too much fuel. However, if your bike can take off in a sprint but lacks overall power, then you've got too much air.
The carburetor consists of four components:
- Pilot Jet - this controls the amount of fuel when idling
- Main Jet - this controls the fuel when you open up the throttle (between 50 and 100 percent power)
- Jet Needle - this controls the fuel when you open and close the throttle (between 20 and 80 percent power)
- Needle Jet - this is what the jet needle pops in and out of (between 15 to 60 percent of throttle
As you can see the carburetor consists of several parts that work, sometimes in unison, to influence the mixture of fuel and air.
Jetting Your Dirt Bike
This is where it gets easy but tricky at the same time. If you're running rich, for example, you'll need to check the main jet, jet needle and needle jet. However, you only need to tinker with the main jet and if that solves the problem you found the easy solution. If it doesn't solve the problem then you'll need to check the jet needle and then the needle jet.
Don't mess with all three jets at once. Change the main jet first and see if that solves the problem. If it doesn't, you'll need to reestablish the main jet to its original position and then move on to the jet needle and so on.
If you change all three you'll never know what jet posed the air/fuel mixture problem, you'll overcompensate to lean and create a bigger puzzle then necessary. Follow the same procedure if your bike runs lean.
Once you get used to jetting your carbs you'll be able to easily adjust the air/fuel ratio based on the variety of factors noted above. Your bike might run rich in the late summer months and then run lean come early spring. Similarly, a trip to the beach might require some adjustment if you last rode in the arid desert. Elevations also mess with jetting because of the dense air. Higher altitude (thinner air) requires less fuel or leaner jetting to run correctly.
If adjustments don't fix your rich/lean running problem, it's time for replacements or you ride in varying geographies requiring constant tinkering with the jetting, consider purchasing a new jet kit. Jet kits provide a selection of jet sizes to make it easy to add/reduce the amount of fuel/air mixture.
Many dirt bike owners go the OEM route for new jet kits however aftermarket jet kits often work just as well if not better. If you need a new jet kit consider the following or try a repair kit.
Update: Despite our best effort sometimes we overlook the obvious and thanks to Brian we fixed our error.
Written By: AndrewT