Inside A Motorcycle Carburetor - Main Jet Tuning

Learn about this little metal piece that allows your motorcycle engine to run wide open

"I'm on the main jet!" Have you ever heard that expression while bench racing with your riding buddies? What the heck does that even mean? Well, after this episode of MC Garage, you'll know. Today we talk about main jet.

For the past few weeks we've been going through a carburetor piece by piece. If you haven't watched the first videos in this series, jump back, and check them out. We will have the links in the description below. We've covered the basic layout of the carb, the float bowl, and the pilot jet. The next piece in the properly running carbureted-motorcycle equation is the main jet.

The main jet is responsible for supplying the fuel that mixes with the air as it makes its way through the intake tract after the pilot jet is done doing its job. Starting at about 20 percent throttle the fuel flows through the main jet. (There is another part that meters that fuel until 80 percent throttle, but we will cover that next week.) When you are wide open, you’re on the main jet.

The main jet is located at the bottom of the carburetor inside the float bowl. It's the larger of the two jets, and it is installed in the needle jet—that part we will be covering next week. If this guy is plugged up, you're in for a full carb cleaning ritual. Everything will usually be clogged if the main jet is plugged up. Check out Ari's video on how to clean a carb.

So we are going to assume you have a clean carburetor so we can get into knowing if your main jet is too rich or lean. First, start with the manufacturer’s recommendation in the service manual. Just like we discussed in last week’s video, there will be recommendations according to temp and altitude. This is where you want to begin.

A “lean” condition means the ratio of air to fuel is too high. This can cause issues from not making full power at wide-open throttle and rpm to detonation and pinging that could damage your engine over time. Lean jetting is the kiss of death for two-strokes with a possibility of a quick meltdown and seizure. Damage can take longer in a four-stroke.

It’s fairly easy to hear pinging on a two-stroke. Think of a metallic tick that sounds like a rattling ball bearing in a tin can as the fuel charge detonates before it should, smashing into the top of the piston too early before it reaches top dead center (TDC). The same sound can be heard from a four-stroke, but the sound is masked by the sound of the valve train. If you hear this, you need to rejet before you do serious damage.

Also, a lean condition will have a crisp throttle response, but the top-end will feel weak and the bike will run hotter than it should. Many times I’ve heard two-stroke racers say it was running so crisp and snappy and then it just stopped. Classic case of a seizure from being too lean. To fix a lean position, go up a jet size and run the bike again; repeat until you find the right size. These numbers may not increase or decrease in increments of one; usually it’s by five or 10.

“Rich” means there is more than the optimal amount of fuel in the air charge. This leads to stumbling and poor overall running. It’s doesn’t lead to a premature death like a lean condition can, so if you must, err on the side of being rich. As the bike moves through the midrange and onto the top-end, the power will stumble and miss. It’s an easier condition to feel and hear for most people. It’s clear it is running bad. Just drop a jet size and retest.

On a two-stroke you can also do a plug chop to check for proper jetting, but this will be a separate video in the future. For now, that’s it for checking the main jet. It’s one of the simpler jets to get dialed in, but if it’s not right, it can cause issues with next week’s topic—the needle jet and needle.