Rejetting Carbs in 3 Simple Steps

15 May

Rejetting CarbsRejetting carbs can be a real hassle. In this post I’ll explain the process of rejetting your carburetors in 3 simple steps:

  1. Main adjustment (main jets)
  2. Fine adjustment (gas needle + air adjustment)
  3. Stationary adjustment (air screw / fuel screw)

Before you start

A carburetor has a lot of parts in it that consist of “soft” materials. That means that carbs will wear out over time. To make sure that your parts are in mint condition, it’s advised to use a carburetor rebuild kit. Since the materials are soft you have to work on them with proper tools and care!

Parts overview:

Carburetors come in a lot of different types but in general they all work the same and have the same parts inside. Below is an overview of the most important parts inside the carburetor:

rejet carbs

1. Main adjustment (main jets)

The most important part of rejetting carbs is the main adjustment, because the main jets are influencing all other adjustments. The main jets regulates the air/fuel mixture when the throttle is fully opened (braaaaaaaaaaaap!!). 1/4 to 3/4 throttle are regulated by the gas needle.

First of all it’s important to have the motorcycle running (how it’s running does not matter). Take it for a small ride (1 or 2 miles) to warm up the engine, open op the throttle and shut it of while riding. Stop the motorcycle and remove the spark plug. The spark plug can tell you what happens inside the cylinder and what the overall operation condition of your engine is:Spark plug color reading

Optimal Spark Plug ColorThe guys from Jets ‘r Us wrote a detailed guide on how to “read” your spark plugs, including pictures: check it out here. Here you’ll find another color chart: spark plug reading. In general you´re looking for a spark plug that is dry and has a coffee color without too much carbon (a little carbon is no problem). Any unusual colors, moisture or other “stuff” should be a red flag. Use the color charts to determine what your plugs tell you.

Rich & lean fuel/air mixture

Next to the spark plug, the overall running condition of your engine can tell you more about the fuel/air mixture. Below is a chart that can help you:

Symptoms of a rich mixtureSymptoms of a lean mixture
Poor power outputPoor power output
Easy cold start, no choke requiredHard to start when cold, excessive choking required
Hard to start when hotHard to warm up the engine
Ragged idle (due to loading up unburnt fuel) also lower RPM than you would expectRough, erratic idle and a higher RPM than you would expect
Smell of fuelBackfiring
Sluggish throttle responseVery “vague” throttle response
When hot: runs worse with choke onWhen hot: runs better with choke on
Uses more fuel than you would expectHesitates when throttle is opened up
Black smoke when acceleration hardBackfiring when you decelerate

After you’ve changed the main jet, take it for another ride and try if you can feel the difference. Sometimes the difference is hard to tell, especially when you only made a minor change in your jet. Go for full throttle and turn the engine off again and stop the motor. Check your spark plug again to see what he tells you. Repeat this process until the spark plug has the right color.

Be careful: most engine jams occur when the main jet is too small. A lean fuel/air mixture will cause the engine to overheat fast. 

2. Fine adjustment (gas needle + air adjustment)

Now you’ve found the right main jets, it’s time for a more fine adjustment.By adjusting the gas needle you control the fuel/air mixture with the same throttle position. The gas needle controls the fuel/air mixture from 1/4/ to 3/4 throttle.

If you open up your carburetor and pull out the throttle slide, you see that the gas needle is kept in place with a little spring clip which fits in one of the grooves and gives the needle a higher or lower position at the same throttle stand: higher means richer, lower means leaner. Most gas needles have 3 or 5 of these grooves and the middle one is the standard position and the position to begin with.

adjusting gas needle

Go for a little spin and try to get some more “feeling” with the throttle response: how does the engine respond when opening up the throttle and what happens when you slowly close it back down. Remember: it’s about 1/4 to 3/4 throttle range. Now go back to your garage and set the gas needle on it’s lowest position (spring clip at highest groove from below). Take it for another ride. You must feel a difference when playing with your throttle; especially when revving it up in first gear.

Now you’ve set 2 reference points so you have some more feeling about the best gas needle position. Go back and try to pick the best spot. It’s very important that the gas response feels natural and logical: if the engine runs better when you’re closing the throttle you have to try another position.

If the response remains slow or unnatural you may have to look for another (thinner) gas needle.

3. Stationary adjustment (air screw/ fuel screw)

Last step is adjusting the air mixture screw or pilot jet. This screw controls the air/fuel mixture when the engine is running stationary and helps the engine start smooth. Working on this little fellow can also indicate that your main jet is too big or small.

There is a difference between an air screw and a fuel screw (or pilot jet). In general al four stroke engines have pilot jets and two stroke engines have air screws. The location of the screw can tell you what your carburetor has: air screws are located on the air intake side, fuel screws will be on the engine side. They work in opposite directions: an air screw controls the amount of air is being delivered (in is rich, out is lean) and a fuel screw controls the amount of fuel (in is lean, out is rich).

We start from the standard position of the screw which normally is: fully screwed in and then two 2 times 360 degrees counter clockwise. Consult your workshop manual to be sure. So in a normal situation you’re engine will start perfectly and run smooth when idling. If not, this screw can help you tweak it a little bit.

First you have to warm up the engine. Adjusting the screw can only be done at low RPM. Turn in the air/fuel screw (clockwise) until the engine begins to stumble and note how much you’ve turned it in (in quarters of turns). Do the same thing but then turn the screw out (counter clockwise). Between these turns (from “stumble to stumble”) you have to “find” the ideal setting with the highest RPM and smoothest running engine. The idle screw (mentioned below) can help you with this.

Idle adjustment

Once your done rejetting your carbs you can do some more fine adjustment on the idle adjustment screw. This screw controls how much RPM you’re engine is making when idling. The idle adjustment screw can also help when you’re working on the air/fuel screw from step 3.

Some tips

  • Don’t use force on any of the parts! You can damage or break things easily!
  • If you have the ability to use a dyno, or know someone with some more knowledge of adjusting motorcycle engines, use it!
  • The main jet size also depends on the altitude and temperature you’re riding!

Rejetting Carbs – Summary

So in short you have to take these 3 steps:

  1. Selecting main jets for full throttle, based on what your spark plugs tell you
  2. Adjusting the gas needle and air mixture screw for half throttle and throttle response
  3. Selecting and adjusting pilot jets for stationary

If you’re doing this for the first time, it really could be a difficult sometimes. It’s a matter of patience, knowledge, experience and feeling. If you’re stuck: don’t give up, get to know your carburetor(s) and work on getting “the feeling”. And remember: you can always ask another rider for help!

I hope this post will help you rejetting your carburetor(s) Good luck!


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