Motorcycle Jet Kits & Carburetion Basics

Fundamentally, increasing fuel flow means installing larger jets in the carburetor. Sounds simple, doesn't it? Trust me, I've been doing it now for more than 30 years, and it really is that straightforward.

Picking the right jet, now that's the hard part. Fortunately that element of the job has been made much easier by the proliferation of jet kits now available. These kits contain all of the bits and pieces you'll need plus detailed instructions, and they are available to fit nearly every current and not-so-current motorcycle I can think of. But this would be a damn short article if we left it at that.

Rather than just show you how to install a jet kit, let me walk you through the basic steps used to jet a bike from scratch. Even though the kits greatly shortcut the jetting process, individual circumstances may dictate some fine-tuning from bike to bike. If you know how to evaluate jetting, you'll know what to look for and what changes need to be made. Since the most popular carburetor in use is the variable-venturi, slide-type carburetor, that's the model we'll use. For the sake of convention we'll assume you have either installed a jet kit and want to make sure the jetting is spot-on or you suspect some jetting changes need to be made.

Presumably you already have some fundamental understanding of a carburetor's internal workings. If you don't, I suggest you bone up on a little theory first. Mr. Bernoulli was a pretty bright guy, and his insights are well worth discovering.

It Just Ain't Running Right

Deciding where the problem lies is the first step. One of the more common mistakes the novice tuner makes is trying to jet based on engine rpm. Throttle position dictates which carburetor circuit is being used at the time, so jetting is always predicated by throttle position, never rpm.

Experienced tuners and riders are pretty good at determining which carburetor circuit is at fault. If you're new to this, a visual aid may prove helpful. A throttle index will show you exactly which circuit is in play at any given time. Using masking tape, place a datum point on the throttle housing. Next use another piece of tape to divide your throttle into quarters. As an alternative you can mark the quarters on the housing and use a colored pin stuck into the grip to indicate the throttle's position.

Your markings should indicate 1/4, 1/2, 3/4 and wide-open throttle, although the last is usually pretty obvious, as is the closed position.

Too often a carburetor jetting chart seems to indicate a very clear delineation among the effects of the various jets and circuits. There isn't; in fact, there is always some overlap, and bear in mind that carburetor fuel-delivery circuits are cumulative. In other words, even when we're running wide open on the main jet, the pilot jet is still supplying some fuel. For this reason we start by addressing the pilot jet first and the main jet last.

The Beginner's Guide to Jetting

Pilot circuit; 0 to 1/4 throttle

Start by bringing the engine up to operating temperature. Open the throttle slightly. Does the engine pick up speed smoothly? If it doesn't, the fuel-mixture screw may be out of adjustment or the pilot jet may be either too small or too large. To determine the pilot screw setting, adjust the idle-speed screw so that the engine is idling slightly fast, somewhere between 10 and 20 percent above the recommended idle speed. Using either the manufacturer's or the jet kit's recommended pilot screw setting as a starting point, turn the fuel screw left or right between 1/4 and 1/2 of a turn. Select the screw position where the engine speed increase is the greatest. Readjust the idle speed to the correct level.

Next, take the bike for a ride, preferably on a relatively quiet, straight section of road. Using your throttle index, run the bike at 1/4 throttle. If the pilot jetting is correct, the engine should run smoothly, and it should be easy to maintain a constant speed without varying the throttle position. If the bike surges or hunts, the pilot jet is probably too small (lean). Other indications that the pilot circuit is lean are popping or spitting through the carburetor when the throttle is opened and popping or backfiring through the exhaust when the throttle is shut. If the pilot circuit is rich, the engine will have a rough idle or may not return to idle without "blipping" the throttle. You may also notice a distinct smell of raw fuel and some eye irritation from the exhaust.

Low- to high-speed transition; 1/4 to 3/8 throttle

Although the throttle cutaway plays a significant role between 1/4 and 3/8 throttle, it is seldom changed when rejetting a streetbike, particularly a cruiser. For that reason I'll gloss over it. Basically the only time the cutaway needs changing is when the transition from the low-speed fuel circuit (pilot jet) to the high-speed circuit (needle, needle jet and main jet) gets seriously out of whack.

Medium throttle opening; 1/4 to 3/4 throttle

The fuel delivery at this stage is controlled by the jet needle and the needle jet. At one time all carburetors came with adjustable jet needles. Once the smog Nazis got involved, fixed-position needles became the standard. Jet kits will usually include new needles that can be raised (to richen mixture) or lowered (to make it leaner) to suit or, less often, shims to change the height of your stock needles.

To evaluate the needle/needle-jet performance, run the motorcycle in second or third gear. Roll the throttle on from 1/4 to 1/2 throttle only. The engine should accelerate cleanly without sputtering or bogging. If so, you're all done with the midrange; if it doesn't, you'll need to do some experimenting to determine if the needle position is too rich or too lean. Assuming you've installed adjustable needles from the kit, begin your experiments by trying some alternative needle-clip positions, perhaps a step richer to begin with. If that makes the situation worse, try a step leaner. If the jetting is stock and problems crop up, it's time to order the appropriate jet kit.