Carb Tune Up Part 2: Jetting and Tuning

So your motorcycle isn’t running so hot, or maybe it is running a little too hot. Maybe it’s sputtering, back firing, bogging down, or not idling. Whatever the symptoms, your motorcycle may be running too lean or too rich and you may need to re-jet your motorcycle. This guide will walk you through the basics of diagnosing and jetting or tuning your carburetor. If you want a handy version of this tuning guide that you can print off and use in your garage, download the Carb Jetting PDF version of our guide.

Diagnosing: How do I know if I need to adjust the jets?

Motorcycles don’t normally need to be re-jetted, so it’s always good to diagnose a few things first to narrow it down to the jets. If the bike has been sitting all winter and you just pulled it out, it may just need a carb clean.

Make sure your carburetor and jets are clean before re-jetting to rule out a clogged jet as the issue. A great way to see if the bike is running too lean or too rich is to check the spark plug(s) after the engine has been warmed up. Warm up the engine and then run in the throttle range that is affected the most and then immediately turn the bike off without letting it idle (unless you’re diagnosing the idle). Pull out the spark plug and if it is black or wet, it’s most likely running rich. If the plug comes out white or ash colored, it is most likely running too lean. The spark plug should be a tan color if it’s operating in the right fuel mixture range.* Bogging down, backfiring, coughing, sputtering, high temperatures, and smoky exhaust could all be signs that your engine is not getting the proper fuel mixture. If these symptoms are more isolated to a particular throttle range, it will be easier to diagnose and fix.

*Since Most gas stations now sell ethanol gasoline, spark plug color can no longer be trusted for diagnosis because of the way it burns, so only use it as a rough guide or if you are using ethanol free gasoline.

Needles, Screws, and Jets

Here’s a chart to help you diagnose what to start adjusting first based on how your engine is performing. Unless you’re very familiar with your carburetor, take care when taking it apart and note where each part goes and be careful not to strip out the screws. (Use the right tipped screw driver!) If your idle needs to be adjusted, first measure how far out your air (or fuel) screw is. To measure it, screw it in until it lightly seats counting as you go. Generally it should be around 1.5 turns out on most carburetors.  Turning the airscrew in clockwise enriches the mixture and turning it out will lean it. If you have a fuel screw (will be on the engine side of the carburetor) instead of an air screw, then screwing it in clockwise will lean out the idle mixture and turning it out counter clockwise will enrich it. If it doesn’t respond to another turn out or in and fix your idle issue, then you’ll probably want to change out the pilot jet. The pilot jet controls your low-speed and idle mixtures. If you change out your pilot jet, make sure to adjust the air or fuel screw if necessary as it controls the amount of air/fuel that is mixed with the fuel coming from the pilot jet system. The slide or throttle valve controls the off idle to one-quarter open mixture. You may be able to find a slide for your carburetor with a different cutaway. More cutaway means it will be leaner (more air) and less cutaway is rich (more fuel). The jet needle controls the amount of fuel delivered to the engine at low to mid throttle and is often adjusted the same time the pilot jet or air screw is. We would recommend that if you need to make the idle mixture richer with the air or fuel screw that you drop the clip on the jet needle one step and if you lean the idle mixture out that you raise the needle clip one step. That should make your throttle transition smoother. The main jet is what controls the mixture at ½ to full throttle. If you’re throttle is bogging down at full throttle, you might need to reduce the main jet size to lean it out. If you’re running hot and get popping or backfire, you might be running lean and need to go up a couple jet sizes. Going up or down to the next main jet size is sometimes not very noticeable, so it’s usually better to move up or down 2 jet sizes at a time when adjusting the main jet initially. When in doubt, it’s always better to be a little rich. (Isn’t that true in life?) Again, if you’re not sure, run it warmed up and. The most frustrating part of tuning a carburetor is knowing what to change, not how to change it. Listen to what your engine says and use this guide as a reference. If you stick with it, you’ll get that engine purring like a kitten soon enough.

There are few things in life more beautiful than a properly tuned engine, especially if you are able to do that yourself!

Carburetor Tuning Tips:

• All of the jets overlap each other in terms of adjustment, so if you adjust one jet, you may likely need to adjust another one that overlaps.
• Keep notes on original jet sizes and the positions everything (pilot screw, needle shim, etc.) is at before you start and keep notes along the way. That way if you royally screw things up, you can always go back.
• When you get jets, buy several sizes above where you are at and several sizes below. That way you won’t have to keep ordering more and your adjustment will go a lot faster.
• New gaskets are a good idea anytime the carburetor is taken apart.
• If you have to adjust your main jet size significantly, it could mean that you have developed an air leak somewhere, so find it and fix it!
• Make sure your carburetor is clean. If you have a clogged jet, you may just need to clean your carb to fix your issue
• Valves can also cause issues with how the engine runs. Make sure your valves are adjusted properly

Download PDF Printable Version of this guide here

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