Below are some general guidelines for changing jets when you make changes on your bike.
When do you need to rejet carbs?
Some common situations in which require rejetting motorcycle carbs include:
- After installing a new exhaust.
- After installing a more restrictive air filter or less restrictive pods.
- After boring your cylinders to a larger size.
- After changing altitude.
This guide serves as a general procedure for deciding which jets to try when changing from stock.
A plug chop will help you determine whether new jets are necessary.
Rejetting the Carburetor Main Jet
The main jet on your motorcycle is responsible for about 3/4 to wide open throttle.
These are found in the center of your carb where the needle enters and closest to the bottom of the float bowl.
They are typically brass and feature the widest opening of all the carb’s jets.
You can find the number of the main jet, which refers to the size of the opening, stamped on the side or top of it.
To tune your main jet, you can perform a plug chop at wide open throttle. If necessary, make changes to the main jet in small increments and retest.
If the spark plug reads lean, go up a size. If the plug reads rich, go down a size.
- Add 2 main jet sizes for a 4 into 2 exhaust.
- Add 4 main jet sizes for a 4 into 1 exhaust.
- Add 4 main jet sizes for open headers.
- Add 2 main jet sizes for a single K & N air filter inside a stock airbox.
- Add 2 main jet sizes if modifying a stock airbox with holes.
- Add 4 main jet sizes for pod filters.
Add up all your changes and then:
- Subtract 2 main jet sizes.
- Subtract 2 main jet sizes for every 2,000 feet above sea level.
Rejetting the Pilot Jet
- Add 1 pilot jet size for every 3 main jet size increases.
- Subtract 1 pilot jet size for every 6,000 ft above sea level.
Remember, these are just general starting points. Every time you make a change you’ll need to re-sync, check your plugs, and observe how the bike operates at different throttle positions.
How do your Rejet Carbs for pod filters?
If you’ve replaced your motorcycle’s stock airbox and filter with pod filters, you’ll typically need to rejet the carbs.
When you use pod filters, the fuel/air mixture will generally be leaner than it was before.
This means, you’ll need to increase the fuel to balance out the increase of air.
Using the guidelines above, here’s how you’d rejet carbs with pods:
- Add 2 sizes to the main jet.
- Move the needle clip down a notch.
- Richen the pilot jet.
Be sure to run a plug chop and make minor adjustments across the different throttle ranges to get your carbs tuned correctly and your motorcycle running properly when using pod filters.
Do you need to rejet carbs with a new exhaust?
Yes, when you change up the motorcycle’s stock exhaust, you will likely need to rejet the carbs.
Different aftermarket exhaust manufactures may have recommendations for the ideal carb jetting to run with their specific exhaust.
Again, it comes down to making minor adjustments and checking the bike’s performance at idle and at different throttle positions.