How to install main jet in YZ400

I bought a new FMF Power Core IV S/A for my '98 YZ400F. FMF recomended installing a richer main jet. I didn't think this was covered very well in the manual, so here's what I learned.

To change the main jet, first shut the fuel off at the petcock. Then remove the vent line from the bottom of the float bowl. You are about to dump a small amount of gas so put out the cigar. It's very tight under the bottom of the carb, but with a quarter inch drive ratchet and a 14mm socket you can remove the drain plug from the bottom of the float bowl. Okay, now there is gas everywhere. Best to just give it a few minutes to evaporate.

Take the new jet you are going to install and clean it with carb cleaner and compressed air. Take a look at it and make sure the passages are clean and clear.

Next get a 6mm socket for your ratchet and look under the carb where you removed the plug. You can just see the bottom of the main jet (brass color, 6mm ). Take your ratchet and remove the "old" jet. If you look on the side you can see a number stamped on it, "175" is stock for the YZ400F.

If you've got big fingers like me the little jets are hard to handle so remove the socket from the ratchet and use it as a handle to hold the jet while you start it. Before you start it, remember the needle jet goes in to the main jet, I get the needle out of the way by having a helper hold the throttle open while I install the Main Jet. CAREFULLY start the jet into the carb, it's brass and very soft, so easy does it. After it starts, thread it in by hand until you feel it bottom. Use the ratchet to tighten it just a little more,easy, just a little does it. This thing is brass and will strip if you over tighten it. Okay the main jets in so have your helper gently close the throttle.

Reinstall the float bowl plug on the bottom of the float bowl and hook the vent line back up.

Turn the petcock on and check for leaks. If it's dry, you're done with the Main Jet and ready to test ride!

Hope this helps,


You can drain the carb gracefully. Find the drainhose on the bottom of the carb, and loosen the allen screw (left side) to let the gas drain out of the tube. Take an old quart oil container and label it OLD GAS and catch the gas. I don't reuse as it gets dirty from the dirt at the end of the tube. I have put some red tape at the end of the line to find it easily next time. Now only a few drops will come out when you take off the 14mm plug. Also if I plan on working on it, I will turn the gas OFF the last 1/8 mile and use up what is in the float bowl. I always drain the carb if I won't be riding it for a few weeks. helps keep the carb clean, and keeps pressure off the fuel valve seal in the carb, which do wear.

i do have a honda but im lookin at a 426 but anyway ive never jetted a carb but ive cleaned it out but wat do people mean when they talk about turns lik 2 turns or 2.75 ?

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now