Carburetor Jetting continued

Correcting Problems, continued

Main Jet

Finally, the main jet portion of the carburetor jetting game. The main jet is the supplier of fuel to your engine at the top end of the RPM range.

The main jet is located in the center of the carb float bowl. The fluid needle that hangs down from the top slide dips into the main jet holder. This area can also be called the emulsion tower.

There are generally two types of main jets, round and hex. The round type look like a large aspirin with a regular screw driver slot. The hex type have a tiny, 6 sided nut shape. Both types should have a number on them somewhere.

My experience has been that if a stock bike has an air filter element changed, the main jet size needs to be increased 3 sizes (170 to 173). If the same bike has the air box removed and air filter pods installed, go 5 sizes larger (170 to 175).

If a motorcycle has had an air filter element changed and a slip-on muffler added, a main jet increase of 5 sizes is a sound start. Say you have gone all out and changed your airbox to filter pods and added a complete drag style exhaust system, from the headpipes back. I would start with a main jet increase of 10. Only a test ride will tell if that is enough.

Carburetor Jetting Tips

Do it yourself carb jetting is not an exact science, it is nearer to an art. Individual motorcycles respond differently to jetting. What worked on your buddy's KZ750 may not work for your Yamaha Royal Star.

Your elevation above sea level and local air quality will also affect the jetting so use these numbers as a guide.

The thinner air at higher elevations will require less fuel to keep the air/fuel ratio constant. I installed a jet kit on a Suzuki Katana 1100 at my shop in the Poconos, then delivered it to a town on the Jersey shore. The bike ran great in the Pocono Mountains but in NJ the bike was running so lean the headers began to glow. I had to move needles and change jets to tune it for the lower elevation.

Fine Tuning or Dialing In

Once you have corrected the major problems with new jets and fluid needle adjustments you still have the option of fine tuning the air/fuel mixture to smooth out your ride.

The air fuel mixture screws are located between the throttle plate on the carburetor and the cylinder head. Look for a screw driver slot on the top or bottom of the carb housing.

One some motorcycles these screws are covered with a tin, aluminum or lead plug. I, personally, use a tiny drill bit to put a hole in the plug. Do not go too deep, just 1/8" or so. Then insert a coarse wood screw and turn to thread it in. Now use pliers or vise grips to remove the plug and later to replace it again. This way you can get a firm grip on it and it can not get lost.

If you turn the screws in, clockwise, you are decreasing the flow of air. This makes your ratio richer. Turn them out, counter-clockwise, and more air can enter.

How do you know when you have it right? You can listen to the bike as you slowly turn the screw. Listen for the engine sound to smooth out.

If you have an infrared thermometer or pyrometer you can check the temperature of each head pipe to ensure that all the cylinders are similarly tuned. The easy to use, non contact handheld infrared thermometers make it easy and safe to identify the cylinder or carb that is giving you problems. You can get them from automotive tool houses. We use a Snap-on meter. It is not necessary, but it's a little more accurate than your burned fingers.

It is not the specific temperature of the cylinder that you are trying to adjust, more so that all cylinders are near equal in temp.

Test Ride

Make sure that you test ride the bike after each carburetor jetting modification. Do not think you can change all the components at once and then test ride. If things are not as you want them to be, it may not be clear which component needs more adjustment.

The exception to this would be if you have purchased a quality jet kit specifically designed for your make and model motorcycle. They have already done the troubleshooting part for you. You should be safe installing the complete kit before your test ride.


All of the carburetor jetting information presented in these two pages is geared towards home tuning of your personal bike. I have purposely left out some of the more technical tuning and the complete topic of multi-carburetor synchronization. I doubt the guy doing a little carburetor jetting at home is willing to buy expensive tuning equipment, so that will be covered in depth on another page.

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