How-To: Tune a Motorcycle Main Jet

What does a main jet do?

A main jet meters fuel at ¾ to full THROTTLE position. Notice I wrote THROTTLE, not engine RPM.

What does a main jet look like? Where is it located?

It will usually be made of brass and gold in color. They can have a round or hex body.  Below are all different types of main jets that are typically found in Mikuni or Keihin carbs. The top 4 pictured are for the XR70 featured in this post/video.

The main jet can usually be found closest to the bottom of the fuel bowl, as shown by the yellow arrow pictured below. It should have the largest orifice of all jets in carburetor.

What do the numbers stamped on the side mean?

Each jet should have a number stamped on its side. Be aware that some low cost jets may not have any numbers stamped on them. As the number increases, so does the orifice, therefore a richer jet.

Why do I have to tune my main jet?

Here are a few reasons why you would need to change out your main jets:

• Modifications to exhaust or air intake (slip on or filter pods)
• Lean straight from the factory (maybe to meet EPA or other requirements)
• Altitude change. You have to run less fuel at higher altitudes and vise versa for lower altitudes.
• Temperature/humidity. When hotter and more humid outside,less fuel is required.

How-To tune a main jet

Don’t reach for tools yet, have a peek in your service manual to see what stock jetting is. Also, don’t order any jets until you have disassembled your carbs and verified what is actually installed. People love tinkering with carbs and sometimes the jetting can be way out of whack for whatever reason. Trust me on this one. 

Before you start tuning the main jet, I like to make sure the pilot jet is sized correctly. Some people start with main jet tuning first, and I guess that is perfectly fine. Consider this though, the pilot jet is flowing 100% after the throttle is open ⅛ or more, so it does contribute to every other circuit, by a small amount. Also, if the pilot jet and pilot air/fuel screw is properly tuned, the plug readings will be more accurate since you won’t have a severely rich or lean condition. 

Since jetting is a trial and error process, it will save time and money if you know whether you need to go richer or leaner. In the video below, the bike had a lean bog due to the aftermarket air pod, which flows more air over the stock airbox. Also, performing a plug chop verified that it was lean. So I went ahead and ordered jets up to 4 sizes larger than what was called out in the service manual.

Step 1 – Warm up the bike to operating temperature. 

Step 2 – Install a brand new spark plug(s). 

Step 3 – Perform a wide open throttle (WOT) plug chop. You’ll notice in this video I ran the bike to top speed 2-3 times so plug will show color. The straightaway should put a load on the engine and a slight uphill is preferred. Pay attention to how the bike runs. Your “butt dyno” is one of the tools in this process. 

Step 4 – Read the plug. You are looking for a brown color on the flat portion of base ring. The base ring should ideally have one full thread turn of color. 

Step 5 – Determine if you are lean or rich based off plug color. If you are unsure, make a change to main jet (make one change at a time!) and retest.

Step 6 – Repeat all steps.

How-To Video

The video at the top of the page shows you the symptoms of a lean main jet, and a properly sized main jet in the video below. Enjoy.

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