Installing a Jet Kit

Original article printed Sport Rider, August 1997

(With Factory Pro Notes added August 19, 2000)  

Tools needed for the job include:
Assorted length and size Phillips and flat-blade screwdrivers ( the long screwdriver may be needed for on-bike adjustment of the idle fuel screw; a long Phillips screwdriver or hex-wrench socket with extension may be needed to reach the manifold clamps); needle nose pliers; assorted ratchets, sockets and extensions (for removal of fuel tank); a float-level measurement tool or small ruler graded in millimeters (optional); and a can of WD-40 and contact cleaner. Tools not pictured are an open end 8mm wrench (for tightening/holding the emulsion tube when changing main jets), a 6mm socket and ratchet and a shop manual. Dialing in the carburetion will take time, so be patient. Most of the jet-kit manufacturers have tech lines to help you with carburetor tuning.
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Installing a performance exhaust system is one of the first modifications people make to their sport bikes. But in order to reap the full benefits of the increased flow (and usually to ensure the engine even runs properly), an aftermarket carburetor jet kit is required. Improved flow at the exhaust side usually results in an engine flowing more air on the intake side, meaning that the carburetors must compensate with more fuel to ensure the correct mixture. Although a carburetor seems incredibly complicated, its inner workings are fairly straightforward, and as long as you take each step in a jet kit installation carefully, you'll be well on your way to improved performance

Factory Pro's Kits are the choice of most of the AMA privateer teams. Factory Pro provides full 4 Gas EC997 Eddy Current Dynamometer Tuning Service  for AMA competitors. The EC997 Dyno is THE primary choice of most teams at AMA.

Removal of the fuel tank is usually your first task. Before removing anything, find the location of the fuel petcock and the main fuel line from the intake manifolds activating the petcock, yet some of these still have an "off" position. Some also require tilting the fuel-tank attachment bolts, tilt the tank up and use needle-nose pliers to loosen the fuel-line hose clamp. After removing the fuel line, use this trick: Set the fuel tank in an old tire, so that it remains level and avoids damage to the petcock. If it's laid on the ground at an angle, fuel may leak out of the overflow vent. Keep track of all the vent hoses leading to the gas tank (write it in a notepad, or label them with masking tape), so you can correctly reinstall them afterwards.


After removing the airbox (being careful to document all hoses leading to and from the airbox), loosen the carburetor manifold clamps. Most take Phillips screwdriver, but some require a hex-key extension for this job. With all manifold clamps loosened, grasp the carbs firmly and remove them, using a "rocking" motion to free the manifolds. Be firm, but don't get too rough. Drain the fuel from the carbs into a container using the drain screw on the bottom of the float bowls.


Before removing the throttle cables, check that nothing has fallen inside the the intake manifolds, and seal them with rag or tape. Do not skip this step. We don't need to tell you what will happen if a bolt or other object falls inside the intake port unnoticed. Removing the throttle cables from the carbs may take a little practice; because of the push/pull actuation, you'll need to turn the linkage with your fingers to free up the cable ends for removal.


Invert the carbs, and remove the float bowls carefully. The main jet (indicated by the screwdriver), which controls fuel flow from the midrange to top-end, is the first component to be changed. Using a flat-blade screwdriver, carefully remove the main jet, taking care not to damage the floats on either side. In fact, try to avoid touching the floats, since you may accidentally bend the float tang, altering float height (next step). When removing the main jet, you may need an 8mm open-end wrench to hold the emulsion tube (which the main jet screws into). Check the jet kit's instructions for replacement size recommendations. Check the condition of the float-bowl gasket, and remove any dirt before reinstalling.


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The majority of (Honda) jet kits don't require you to adjust the float height; call the manufacturer ahead of time to see if your application may require this adjustment. If so, you'll need a height measurement tool like the one pictured (available from most Honda dealers or direct from Factory Pro), a pair of vernier or digital calipers or a small ruler with millimeter markings. Make sure the bottom edge of the float bowl is level, and carefully measure the distance from the edge of the float. Don't be a hamfist here; the float mechanism is very delicate. Adjustments are made by carefully bending the float tang (located at the float's pivot point),. either by gently pushing downward the float, or bending the tang manually.
Note! The pic shows the carbs UP SIDE DOWN! They should only be tilted! The float valve spring should NOT be compressed!

(Factory Pro Notes:
It is extremely important that you DO NOT compress the float valve spring loaded plunger when measuring, except on the 95-97 ZX6r, where you DO compress and bottom out the float valve spring.
Note: Even though the above picture shows the carbs totally inverted - with all the weight of the floats compressing the float valve spring, we all know that the carbs should be only "tilted" so that the float "tang" touches and closes the float valve, but does not compress the float valve spring on the little plunger! that is in the float valve!

Most jet kits require changing the needles and slide springs, which control lower-to-upper midrange carburetion. When removing the carb diaphragm covers, some carbs have a small O-ring on the diaphragm's edge (indicated by the screwdriver). Take care not to loose these, otherwise the slide will not work properly. Be careful not to damage the diaphragm/slide assembly/jet needle during removal. Note the fitment of the slide spring on the diaphragm cover. Incorrect installation of the accessory spring will cause slide malfunction, as well. Clean off any dirt in the diaphragm seal groove. mc_art06_8.jpg (16914 bytes)

(Factory Pro Notes: If the original needle had a thin steel or brass shim washer below the needle clip or spacer, unless the Carb Recal Kit specifically states to remove it, you will use the thin metal spacer with the Factory Pro needle. If the original needle has a 2.5mm / .100" plastic spacer under the clip, you will ALWAYS use that with our needle!)

mc_art06_9.jpg (17644 bytes) When replacing the needle (protruding from beneath the slide), handle the slide assembly carefully to avoid damaging the diaphragm. The needle is usually held in place with a plug of some sort; check your jet kit instructions for proper removal procedure. When removing the needle, note that there may be a tiny washer that sits underneath the top of the needle. Keep the washer, as you may be needing it later when dialing in the needle position for midrange tuning.


The main advantage of a jet kit is the ability to alter the needle "height" to dial in carburetion for various applications. Note that the aftermarket needle (top) has a circlip, which can be set in any of the grooves in the top portion of the needle, while the stock needle (bottom) has no adjustment. Check your jet-kit instructions for proper "base" clip setting. Note that the positions are counted from the top of the needle down: The "number three position" is the third groove from the top. The easiest method for installation/removal of the circlip is to place it in the desired groove, and gently force it in, by pressing the needle and clip against a flat surface. Place the open end of the circlip against a flat surface to remove. Be careful while handling the needles, as they are precision-machined pieces; bending them even the slightest amount renders them useless. mc_art06_10.jpg (12816 bytes)

Factory Pro Notes:
A better description of a jet kit is a supply the parts necessary to properly tune a particular make / model / with the listed mods (like exhaust system, air filter, etc.).
Is the needle the most important part of the kit? Very important, but not really the only item that reduces power output. R&D time is distributed throughout the entire package, like main jet stagger, pilot jet size, float height / fuel level settings, emulsion tube design and additional tuning information. The needle is very important, but often, more work is involved in the other items that must be changed to develop an assembly of parts that will produce a desired fuel curve that, with the correct main jet, will produce best power.|
How much time for each component and the total kit R&D? It's a judgment call and is the result of the accuracy of your test equipment and the designer's curiosity, creativity and stubbornness. We've spent as much as 6 weeks, as on the 95-97 ZX6r (virtually EVERY Kawasaki race team ended up with that kit in the US) and 5 months to properly design and manufacture the kit that eliminated the on/off throttle problem for the 2000 model ZX9r's. We ARE stubborn.
Why does the other carb kit manufacturer sell a cheaper kit that does nothing but aggravate the on/off throttle problem? Not because the don't want to fix it, but that their best dynos (high inertia dynos with an eddy current brake stuck on the end of a 900# drum) simply aren't able to quantify or even see the problem!

mc_art06_11.jpg (12937 bytes) After setting the clip to the desired groove ( the washer that you saved from the stock needle setup can be used for half-step increments between grooves), carefully reinstall the needle, making sure the needle holder is correctly set in the slide. When reinstalling the slide assembly, ensure the needle fits into the emulsion tube as the slide moves into place. The diaphragm's edge must fit into the groove atop the chamber. Don't forget the O-ring (step 8) if needed. Fit the accessory spring ( if supplied in the kit), and carefully install the cover, making sure that the diaphragm's edge is "slotted" correctly.


The final modification is to adjust the idle fuel screw. On this particular setup, the screw is accessible with a special tool included in the kit. On some older carbs, however, the screw is covered with an aluminum plug, which requires a drill to remove. Check your kit instructions for the correct procedure.

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Factory Pro Notes:
On US Honda's, the fuel screw is usually adjusted with a "D" wrench and it is not covered by a cap.
On some older style Keihin carbs, as on the NT650, the fuel screws are located just below the throttle shaft pulley, covered by an aluminum cap and are adjusted with a flat bladed screwdriver.
On Kawasaki Keihin carbs, the fuel screw is in the same location as the cbr900 carbs shown above, covered by an aluminum cap and are adjusted with a flat bladed screwdriver.
On modern Mikuni carbs, the fuel screw is also located in the same place, covered by an aluminum cap and are adjusted with a flat bladed screwdriver.

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When reinstalling the carb assembly, spraying WD-40 or other light lubricant onto the manifolds eases reassembly. Of course, make sure to remove the intake port covers before installation. Hooking up the throttle cables may take a little practice: you'll probably need needle-nose pliers to insert the cable end into the throttle linkage drum. Ensure that the manifolds are correctly installed on the intake port spigots (or that the carbs are fully inserted into the rubber manifolds, if removed in that manner), and tighten the manifold clamps. After reinstalling all components, check that the throttle works correctly without binding when moving the bars lock-to-lock.

Factory Pro Tuning
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