How to Install a Jet Kit

Instructions on Jetting

Despite the fact that carburetors are fast fading from the scene, there are literally millions of them still in use. I’ve no doubt they’ll remain so long after I’ve gone to a home for doddering old mechanics. In the main, most carburetors work fine most of the time, but occasions sometimes arise when a little surgery is needed. Hence the popularity of the jet kit. While re-jetting a carburetor may seem like a daunting task—especially if your only contact with one has been turning a twist grip—it’s actually straightforward. Here’s the 411.

Installing a jet kit, dyno kit
DynoJet’s kits include a free dyno voucher, as well as an assortment of jets and a drill bit and sheetmetal screw to remove the pilot screw’s anti-tamper plug.Photography by Mark Zimmerman

What's a Jet Kit?
This may sound like an obvious question, but if you've never purchased one, how would you know? A jet kit is a comprehensive assortment of jets that allows you to reconfigure your carburetor's fuel delivery either to suit some specific purpose such as the installation of an aftermarket exhaust or airbox kit, or to correct some deficiency in the way the bike runs from the factory. As a rule, along with the hardware, you'll receive some pretty thorough instructions and in some cases a credit slip for a free dyno run so you can be certain you've selected the optimal combination of jets for your application. Be aware that not all kits include the same pieces. For example, one manufacturer's kit for a given model might include pilot jets, while another's may not.

Why Use One?
The big advantage in using an over-the-counter jet kit is that all the homework has been done for you. Prior to the advent of kits, jetting was a by-guess-and-by-golly occupation. Many times you'd have to special order things like needles and needle jets. If you guessed wrong, it was back to the parts counter, and you'd have to wait weeks until the new stuff arrived. Jet kits remove the guesswork, and because they contain adjustable needles and a variety of jets, they allow you to fine-tune the fuel/air ratio to your exact requirements. Included in most kits is also a tech number with a real, live person on the other end to answer any questions that may arise. In most cases, there is also excellent online information available. In particular, the Factory Pro Web site ( is very good—an hour or two there can teach you an awful lot about how carburetors work and how to tune them.

What Won't a Jet Kit Do?
Obviously, a jet kit won't repair a damaged engine. If your mill is low on compression, has a cracked intake boot or worn spark plugs, you can install jet kits until you're blue in the face and they won't change a thing. By the same token, if your otherwise properly running bike suddenly develops a case of "won't-idle syndrome" or manifests some other carburetor malady all by itself, chances are good that the problem is the result of something ingested, like water or a piece of dirt that's made its way into one of the jets. If that's the case a jet kit won't help nearly so much as a good cleaning.

Installing the Kit
Carburetors are no more complicated than any other mechanical device. They do, however, have some small, delicate parts, and as such are vulnerable to rough handling and contamination from dirt. So you'll need to work on them with deliberation and in a relatively clean environment. Essentially, you'll need a clean, dry place to work, preferably with a nice, solid workbench, where you can lay out the parts and a selection of hand tools. A can of aerosol carburetor cleaner and silicone spray or WD-40 will come in handy, too.

The job can be broken down into four steps: removing the carburetors; disassembling them; installing the kit and reassembling the carburetors; and reinstalling them on the bike. While the design of a few bikes will allow installation of a jet kit with the carburetors in place, that’s a rarity. Especially since 90 percent of the time, the job entails drilling out the anti-tamper plugs placed over the pilot jet adjusting screw (see "Drilling Out Anti-Tamper Plugs" below for details).

The first job will probably be to remove the carburetors from the bike. Your shop manual will detail the exact procedure, but as a rule you’ll need to remove the fuel tank, remove or disconnect the airbox-to-carburetor hoses and then remove the carburetor cables before loosening the carburetor manifold hoses and pulling the carbs loose. The carburetor manifold clamps usually have smallish, easy-to-strip Phillips-head screws holding them in place. A sharp number 1 or 2 Phillips and a little elbow grease should get them loose, so don’t be afraid to bear down. If any become truly buggered, replace them during reassembly. As an alternative, many foreign-car specialists stock thin worm drive clamps in the appropriate diameters with either Allen-head or slotted screws that are more attractive and somewhat easier to work with than the cheesy OEM stuff. Make sure to mark the location of any fuel, water, vacuum or emission lines you’ve removed before you disconnect them from their ports as well as the routing of throttle cables. You’ll also find it easier to drain the float bowls with carburetors still in place as opposed to sloshing fuel all over yourself by removing them full.

Removing the carbs from their manifolds can be a little tricky, as the rubber tends to glue itself to the carbs. If the carbs stick, try rocking them back and forth while spraying a little silicone or WD-40 at the joint. Once the carbs are off (don’t forget to plug the intake and airbox openings with clean rags) and the float bowls drained, it’s a by-the-numbers exercise.

Some bikes have more carburetors than others. If you’re working on multiple carbs, I suggest you complete all jetting changes first, then flip the carbs over and make all needle/spring changes, just to avoid confusion.

Installing a jet kit, dyno kit
1.Photography by Mark Zimmerman

1. Start by removing the float bowl. You can expect the screws to be tight and easy to strip. I like to replace them with Allen-head screws (available at most hardware stores) on reassembly.

Installing a jet kit, dyno kit
2.Photography by Mark Zimmerman

2. Before going any further, make sure you know which jets go where. The main jet should be in the center of the bowl, with the pilot jet off to one side. Be careful around the floats; rough handling could change the setting, which upsets the jetting. If the floats need resetting, the instructions and your manual will show you how.

Installing a jet kit, dyno kit
The pointer indicates the main jet; the pilot jet lies beneath it. (Step 3)Photography by Mark Zimmerman

3. Select the appropriate jet(s) from the kit and install them. Use a wrench to prevent the emulsion tube (sometimes called the discharge tube or main jet holder) from turning when the old jet is removed. A loose emulsion tube can create hard-to-diagnose running problems, so make sure it's snug. If for some reason you remove the tube, watch out for a small brass tube or "pill" located above the emulsion tube in some carbs. If the tube is removed, the pill may drop out. Since the pill acts as a guide for the needle, it has a chamfered hole on one side and a straight hole on the other, and it must be positioned correctly with the chamfer toward the top of the carburetor.

4. Reinstall the float bowl and loosen the diaphragm/slide cap screws.

Installing a jet kit, dyno kit
5.Photography by Mark Zimmerman

5. Remove the cap and spring, followed by the diaphragm/slide assembly from the carburetor.

Installing a jet kit, dyno kit
This particular carburetor uses a plastic retainer that locks into grooves to secure the needle. The small hex is used to rotate the lock ring so the needle can be removed. (Step 6)Photography by Mark Zimmerman

6. Remove the OEM needle from the slide. There are several ways to retain that needle, and in some instances the retainer may be screwed down. If that's the case (check the manual), it'll be easier on you if you break the screws or retainer cap loose before removing the slide from the carburetor bore. The needle may also have small spacers or shims either above or below it, so don't just tip the slide upside down and give it a shake.

Installing a jet kit, dyno kit
The instructions will detail where the needle clip and any shims should be placed. (Step 7)Photography by Mark Zimmerman

7. The instructions will provide needle settings. Most kits contain a new needle that's adjusted with an "e"-type clip. Place the e-clip in the correct groove and secure it by placing the rounded portion of the clip on a hard surface and pushing down. You should hear a distinct "click" as it slides home. Be sure to grasp the needle as close to the non-tapered end as possible; it's delicate and if bent will be junk. Make certain any required spacers and shims are installed correctly. Some kits use spacers above the needle clip to hold it in place; some use shims below the clip to provide a fine adjustment.

Installing a jet kit, dyno kit
Press the clip into place with the closed end against a hard surface; push down firmly.Photography by Mark Zimmerman

8. Position the needle in the slide (a dab of grease will prevent any shims from dislodging) and gently drop the slide back into the carburetor. Make absolutely certain that the needle enters the discharge tube (a little jiggle won't hurt) and the diaphragm is properly seated in the groove. A little grease or Vaseline (I know there's a joke here somewhere) should keep it in place.

9. Install the new spring if one was provided, making sure it's seated in the cap. Install the cap and screw it down evenly.

10. All that's left is to reinstall the carbs and road test it; if you really want to make sure it's perfect, head over to the local dyno.

Installing a jet kit, dyno kit
Finally, take it to the dyno!Photography by Mark Zimmerman

Tech Tip: Right Angle

Unless you’ve spent a lifetime crawling around greasy rolling stock, you’d never suspect how many different types of fittings there are. Since I’ve only reached the geezer stage of my life, I’m sure there are a few I haven’t come across, but I’d say there are at least two dozen I’m familiar with. Luckily, the majority of motorcycles use only the type of grease fitting known as a Zerk—the nearly universal bulb-type fittings used on everything from printing presses to bulldozers. They are especially common in automotive and motorcycle applications, so you all probably know what they look like.

Although the business end of a Zerk fitting (where the grease gun plugs in) is a standard size, the fittings are available in a variety of angles. Straight fittings are most common; these work well when you have room to plug in your grease gun and whatever you’re working on is positioned on a lift, but not so well when you’re down on hands and knees trying to grease your motorcycle in the driveway. At those times a straight fitting may prove awkward to reach, especially if you’re using a grease gun with a rigid tube. There are two solutions. The first is to trot down to your local auto-parts store and find a fitting better suited to your needs; make sure you match the threads to the old one. Imported bikes normally use a metric thread, while American bikes use SAE threads. Solution Two is even easier. Although you’ll still have to hoof it to the parts store, what you’ll look for instead is a right-angle, grease-fitting adapter. The adapter plugs onto the end of your standard grease gun so you can use it at right angles to the fitting. This makes it a lot easier to grease things like suspension linkage while the bike’s on its wheels, and as we all know, a greasy suspension is a happy suspension.

Jet Kit Do's and Don'ts

  • Do your homework.
    Spend a little time researching what the jet kit contains and whether that kit is appropriate for your needs.

  • Do set aside a clean, quiet place to work.
    If your shop is under an apple tree, this is one job you might want to farm out.

  • Do take the time to read the manual beforehand.
    Make sure you understand how to remove and replace the carburetors and to perform necessary adjustments.

  • Do be patient.
    Carburetor work can be finicky, and it's easy to get off track. If you take your time and complete each step before moving on to the next one, you shouldn't have any problems. Get distracted and you're liable to spend the whole weekend just trying to get the bike to run.

  • Don't expect miracles. A jet kit isn't a magic bullet, and in many cases experimentation may be needed before you hit that golden combination. In some cases you may have to settle for a that's-as-good-as-it'll-get solution.

  • Don't second-guess the instructions.
    The guys writing them have installed more jet kits than you've had hot meals. If they tell you to initially adjust the pilot screw to 4.5 turns, do it. If you later find out it runs best at 3.5 turns, then good for you—but don't assume anything that isn't in the instructions.

  • Don't throw anything away.
    Keep the OEM parts and all paperwork that came with the kit, especially if the kit was installed along with open pipes and/or airbox modifications. The bike's next owner may prefer a stock setup.

  • Don't be afraid to contact the manufacturer's tech line with questions.
    These guys have run across most of the problems you're likely to encounter and will be able to steer you in the right direction should you run into any snafus. If you find a kit that doesn't have a line for tech support, take a pass.

  • Don't panic if something goes wrong.
    Carburetors are simply mechanical devices, and there is nothing particularly mysterious about them. If something fouls up, retrace your steps until you find the problem.

Drilling Out Anti-Tamper Plugs

Although your carb’s pilot screws are adjusted at the factory, chances are pretty good at some point you’re going to need to readjust them. There are two types of screws out there. Type 1 is normally accessible, but requires a special tool to adjust it. Sometimes the tool is included in the jet kit, but when it isn’t you can order it through the manufacturer or the aftermarket. Type 2 uses a standard slotted-screw adjuster, but has an anti-tamper plug placed over it at the factory. If your bike has the latter, your first step in the tuning process will be to remove it.

  1. Locate the plug and use a center punch to dimple it as close to dead center as possible.
  2. Drill through the plug. A few wraps of tape on the bit will prevent it from breaking through and wiping out the adjusting screw.
  3. Screw in the sheetmetal screw.
  4. Pull out the plug using a Vise-Grip.