Tuning your Carburetor - Idle and Main Jet Circuit Tuning


The carburetor we'll be rejetting is a Constant Velocity (CV), carburetors one most motorcycles will use the same procedure. Obviously, other manufacturers carburetors are going to differ in the procedural sense, but the concept is the same: Drop the float bowl, change the jets, then put everything back together. Pretty straightforwards, actually. In fact, the hard part is knowing which jet to change. Is the main jet too rich or too lean? What about the intermediate jet? How do I dial in the idle mixture and idle speed settings? Seeing no rejetting tech piece would be complete without this important information, I'll also tell you haw to know what to put in and the order you should change the jets.

Tuning the Pilot Circuit

The first step in tuning this, or any, carburetor is to get the idle circuit correctly adjusted. (after all, if the motorcycle doesn't start and idle, it's pretty hard to tune anything else.) That means adjusting the idle mixture screw for best idle. Keihin sets the idle mixture (air) screw during assembly and seals the screw under an aluminum plug to prevent an owner from changing the settings. (Most Honda motorcycles use Keihin carburetors.) IF the aluminum plug has been removed (which is done by carefully drilling a small hole in the plug, threading in a small screw, and then pulling out the plug), gently turn the adjusting screw all the way in until it bottoms. Then turn it two and a half turns out from the fully closed (bottomed) position. Next, start the engine and bring it up to operating temperature.
With the motorcycle in the vertical position and the idle near 1,000rpm, turn the idle mixture screw out until the idle either slows or becomes irregular. Now turn the screw out until the engine again slows or begin an irregular idle, counting the number of turns between the too rich and too lean positions. Then set the idle mixture screw midway between the two positions. You can refine this setting later if you want, but this will be very close to the perfect idle mixture setting.

As you continue tuning the carburetor, the mixture setting is going to change if you change anything else in the carburetor. So, if you end up replacing the main jet and changing or shimming the needle, you'll need to retune, the idle circuit. Luckily, the idle circuit is the easy part and is relatively easy to accomplish. Our idle mixture screw has a knurled knob installed on it. This makes it very easy to adjust the mixture. With the stock mixture screw, you'll need a thin, short screwdriver to reach the screw in its cavity behind the carburetor. Practice and patience (with a good tolerance for heat) is the key here.

Note: If the best idle is achieved with the idle mixture screw less than a quarter turn out, the pilot jet is too large and will need to be changed to a smaller (leaner) one. If the idle mixture screw is more than three turns out, the pilot jet is too small and will need to be changed to a larger one (richer).

Tuning the Main Jet Circuit

This is where the most confusing seems to be. The main jet really doesn't come into full play until the last quarter turn of the throttle. When the needle rises in the tube far enough that the gap is equal to or greater than the size of the main jet, the main jet becomes the metering device for fuel flow. This is where the meat of the power lies. At full throttle, the amount of fuel pumping into the engine can be startling. If you've ever seen a motorcycle equipped with a velocity stack on the dyno, the amount of fuel getting sucked into the engine is pretty impressive.

The number stamped onto the jet is the jet's size. This size can be in millimeters, thousandths of an inch, flow rate, or a number of other interesting units of measure. Generally speaking, the larger the number, the bigger the jet, and the richer it will be.

Tuning the main circuit is easy enough. You need to understand the rich and lean conditions, also you need to set a distance on a clear road section – which, in combination with somebody operating a stopwatch, will operate as a homemade dyno. The size of the main jet will not change dramatically as you modify your engine. For example, if you run a 185 main jet in a stock bike, and you swap out the exhaust pipes for a freer-flowing sporter set, a 190 will likely do the trick. Of course, a shop that does this everyday may be willing to let you in on the 'big tuning secret' and tell you which yet size will work the best with the combination of parts you have... From our experience, tuning a carburetor is all about taking the time, and test it and test it and test it more – record your findings and try another needle and test it and more testing until boredom... Tomorrow we look more closely to tuning the jet needle and the lean and rich condition....
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