Installing a Jet Kit (2022)

Installing a Jet Kit

Original article printed Sport Rider, August 1997

(WithFactory Pro Notes added August 19, 2000)

Tools needed for the job include:
Assorted length and size Phillips andflat-blade screwdrivers ( the long screwdriver may be needed for on-bike adjustment of theidle fuel screw; a long Phillips screwdriver or hex-wrench socket with extension may beneeded to reach the manifold clamps); needle nose pliers; assorted ratchets, sockets andextensions (for removal of fuel tank); a float-level measurement tool or small rulergraded in millimeters (optional); and a can of WD-40 and contact cleaner. Tools notpictured are an open end 8mm wrench (for tightening/holding the emulsion tube whenchanging main jets), a 6mm socket and ratchet and a shop manual. Dialing in thecarburetion will take time, so be patient. Most of the jet-kit manufacturers have techlines to help you with carburetor tuning.
Installing a Jet Kit (1)

FactoryPro's US / Canada Toll free Tech Support
1 (800) 869-0497

Installing a Jet Kit (2)Installing a performanceexhaust system is one of the first modifications people make to their sport bikes. But inorder to reap the full benefits of the increased flow (and usually to ensure the engineeven runs properly), an aftermarket carburetor jet kit is required. Improved flow at theexhaust side usually results in an engine flowing more air on the intake side, meaningthat the carburetors must compensate with more fuel to ensure the correct mixture.Although a carburetor seems incredibly complicated, its inner workings are fairlystraightforward, and as long as you take each step in a jet kit installation carefully,you'll be well on your way to improved performance

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

(Video) How to install a MOTORCYCLE JET KIT

Removal of the fuel tank is usually your first task.Before removing anything, find the location of the fuel petcock and the main fuel linefrom the intake manifolds activating the petcock, yet some of these still have an"off" position. Some also require tilting the fuel-tank attachment bolts, tiltthe tank up and use needle-nose pliers to loosen the fuel-line hose clamp. After removingthe fuel line, use this trick: Set the fuel tank in an old tire, so that it remains leveland avoids damage to the petcock. If it's laid on the ground at an angle, fuel may leakout of the overflow vent. Keep track of all the vent hoses leading to the gas tank (writeit in a notepad, or label them with masking tape), so you can correctly reinstall themafterwards.Installing a Jet Kit (3)Installing a Jet Kit (4)

.

After removing the airbox (being careful to document allhoses leading to and from the airbox), loosen the carburetor manifold clamps. Most takePhillips screwdriver, but some require a hex-key extension for this job. With all manifoldclamps 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 thecarbs into a container using the drain screw on the bottom of the float bowls.Installing a Jet Kit (5)
Installing a Jet Kit (6)Before removing the throttle cables, checkthat nothing has fallen inside the the intake manifolds, and seal them with rag or tape. Donot skip this step. We don't need to tell you what will happen if a bolt or otherobject falls inside the intake port unnoticed. Removing the throttle cables from the carbsmay take a little practice; because of the push/pull actuation, you'll need to turn thelinkage 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 totop-end, is the first component to be changed. Using a flat-blade screwdriver, carefullyremove the main jet, taking care not to damage the floats on either side. In fact, try toavoid touching the floats, since you may accidentally bend the float tang, altering floatheight (next step). When removing the main jet, you may need an 8mm open-end wrench tohold the emulsion tube (which the main jet screws into). Check the jet kit's instructionsfor replacement size recommendations. Check the condition of the float-bowl gasket, andremove any dirt before reinstalling. Installing a Jet Kit (7)
(Video) Jet kit installation gone bad. Don't make this mistake on installing a jet kit!

Installing a Jet Kit (8)

The majority of (Honda)jet kits don't require you to adjust thefloat height; call the manufacturer ahead of time to see if your application may requirethis 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 ofvernier or digital calipers or a small ruler with millimeter markings. Makesurethe bottom edge of the float bowl is level, and carefully measure the distance from theedge 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 pivotpoint),. 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!

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

Most jet kits require changing the needles andslide springs, which control lower-to-upper midrange carburetion. When removing the carbdiaphragm covers, some carbs have a small O-ring on the diaphragm's edge (indicated by thescrewdriver). Take care not to loose these, otherwise the slide will not work properly. Becareful not to damage the diaphragm/slide assembly/jet needle during removal. Note thefitment of the slide spring on the diaphragm cover. Incorrect installation of theaccessory spring will cause slide malfunction, as well. Clean off any dirt in thediaphragm seal groove.Installing a Jet Kit (9)

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

Installing a Jet Kit (10)When replacing the needle (protruding from beneath theslide), handle the slide assembly carefully to avoid damaging the diaphragm. The needle isusually held in place with a plug of some sort; check your jet kit instructions for properremoval procedure. When removing the needle, note that there may be a tiny washer thatsits underneath the top of the needle. Keep the washer, as you may be needing it laterwhen dialing in the needle position for midrange tuning.
(Video) How to install a Jet Kit Honda VT750 #how2wrench #jetkit #howtoinstallajetkit #dynojet vt750dc
The main advantage of a jet kit is the ability to alterthe needle "height" to dial in carburetion for various applications. Note thatthe aftermarket needle (top) has a circlip, which can be set in any of the grooves in thetop portion of the needle, while the stock needle (bottom) has no adjustment. Check yourjet-kit instructions for proper "base" clip setting. Note that the positions arecounted from the top of the needle down: The "number three position" is thethird groove from the top. The easiest method for installation/removal of the circlip isto place it in the desired groove, and gently force it in, by pressing the needle and clipagainst a flat surface. Place the open end of the circlip against a flat surface toremove. Be careful while handling the needles, as they are precision-machined pieces;bending them even the slightest amount renders them useless.Installing a Jet Kit (11)

FactoryPro Notes:
A better description of a jet kit is a supply the parts necessary to properlytune a particular make / model / with the listed mods (like exhaust system, airfilter, etc.).
Is the needle the most important part of the kit? Very important, but not reallythe only item that reduces power output. R&D time is distributed throughoutthe entire package, like main jet stagger, pilot jet size, float height / fuellevel settings, emulsion tube design and additional tuning information. Theneedle is very important, but often, more work is involved in the other itemsthat must be changed to develop an assembly of parts that will produce a desiredfuel 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 calland 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 95-97ZX6r (virtually EVERY Kawasaki race team ended up with that kit in the US) and 5months to properly design and manufacture the kit that eliminated the on/offthrottle problem for the 2000 model ZX9r's. We ARE stubborn.
Why does the other carb kit manufacturer sell a cheaper kit that does nothingbut 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 stuckon the end of a 900# drum) simply aren't able to quantify or even see theproblem!

Installing a Jet Kit (12)After setting the clip to the desired groove ( the washerthat you saved from the stock needle setup can be used for half-step increments betweengrooves), carefully reinstall the needle, making sure the needle holder is correctly setin the slide. When reinstalling the slide assembly, ensure the needle fits into theemulsion tube as the slide moves into place. The diaphragm's edge must fit into the grooveatop the chamber. Don't forget the O-ring (step 8) if needed. Fit theaccessory 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. Onthis particular setup, the screw is accessible with a special tool included in the kit. Onsome older carbs, however, the screw is covered with an aluminum plug, which requires adrill to remove. Check your kit instructions for the correct procedure.

Installing a Jet Kit (13)

(Video) JD Jet Kit Install and 3X3 Mod DRZ400SM Supermoto from Thumper Talk

FactoryPro 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 arelocated just below the throttle shaft pulley, covered by an aluminum cap and areadjusted with a flat bladed screwdriver.
On Kawasaki Keihin carbs, the fuel screw is in the same location as thecbr900 carbs shown above, covered by an aluminum cap and are adjusted with aflat 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.

Installing a Jet Kit (14)

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

Factory ProTuning
1 (415) 883-5620

FAQs

How do I know if my jets are too big? ›

Typical Lean Conditions:

(A too-large main jet also mimics this symptom.) - The engine runs hot, knocks, pings and overheats. - The engine surges or hunts when cruising at part-throttle. - Popping or spitting through the carb occurs when the throttle is opened.

How do I know if I need to Rejet my carb? ›

A black, sooty-looking spark plug means you're running rich with too much fuel in your mix. If you see white residue, that means you're running lean with too much air. In both cases, you need a rejet.

Do you screw pilot jet all the way in? ›

All jets are fully seated all the time by screwing them in all the way. If you replace a jet to try a different size then the new jet gets screwed in all the way until fully seated. The pilot screw gets tuned for effect but 2 1/2 turns out from gently seating it is a good starting point.

How do you install a carburetor jet kit? ›

Slide inside the cards. Being careful not damage the needle. And then we are going to take our

Does pilot jet affect idle? ›

Does the pilot jet affect idle? Yes, the pilot jet definitely affects the idle. You don't need to adjust the idle screw until you have properly adjusted the pilot jet circuit and air screw or fuel screw.

Does main jet affect needle? ›

Fuel comes through the main jet and into the needle jet. So the main jet does affect the needle, especially as the throttle opening increases. The needle is attached to the slide and slides up and down within the needle jet. The needle is tapered, like a needle, and has a smaller diameter at the bottom than at the top.

How do I know if my carburetor is rich or lean? ›

Q: How Do You Tell if a Carburetor Is Rich or Lean? A: One way to tell for sure is by "reading" the spark plugs. If the plug tip is white, the mixture is lean. If it's brown or black, it's rich.

When should you increase pilot jet size? ›

What are some indications of a rich pilot jet and/or screw setting? If your air screw is out further than 2.5 turns, you need a leaner pilot. If your fuel screw is in further than 2 turns, you need a leaner pilot. Off idle heavy feel or sputter.

Is the pilot jet the same as idle jet? ›

An idle or pilot jet is a component of a carburetor.

It affects mixture at idle and just off idle. The main jet affects mixture at off idle all the way to wide open and takes over mixture of the AFR at higher engine/intake tract airflow speeds.

What does raising the needle jet do? ›

Raising or lowering of the needle just raises and lowers the sloping line, making fueling richer or leaner “on the needle,” which is from roughly 1/4 to 3/4 of throttle slide lift.

How do you adjust a jet needle? ›

Inside A Motorcycle Carburetor - Needle Height Tuning Adjustment

What does a Stage 1 jet kit do? ›

Stage 1 kits are fully adjustable to allow the use of a well designed aftermarket pipes and stock replacement air filters like K Stage 1 kits are designed to improve throttle response and drivability and in most cases power increases of approximately 5% throughout the entire power range can be achieved.

How does a jet kit work? ›

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.

What does a Dyno Jet kit do? ›

Dynojet Jet Kits increase power and smoothness throughout the entire rev range, while maintaining optimum fuel economy. Each Dynojet Jet Kit includes comprehensive instructions which show step-by-step installation and testing procedures to ensure a perfect setup every time.

What does pilot jet do in carb? ›

The pilot jet handles the mixture from idle to 15 to 20 percent throttle. Air comes in through the front of the carburetor drawing fuel up through the pilot with a vacuum that is created as the air flows through the pilot circuit.

Does pilot jet affect top end? ›

Changing the main jet size will adjust the top end of the operating range from half to full throttle. Adjusting the needle jet size, jet needle, and needle clip position will affect the mid range operation.

Which way do you turn a mixture screw to lean? ›

On most carburetors, turning the mixture screw in (clockwise) leans the mixture, while counterclockwise (out) enriches the mixture. Initially, if the engine stumbles or the vacuum drops when turning the mixture screw in, turn both screws out about a -turn and evaluate the results.

How do I make my motorcycle run leaner? ›

The 6 main ways to fix a motorcycle running rich are to:
  1. Adjust the air to fuel ratio.
  2. Check if the needle is sticking in the carburetor.
  3. Check for stuck open floats.
  4. Clean your air filter.
  5. Check for stuck open butterfly valves.
  6. Change your O2 sensors if your motorcycle has them.

What jet controls idle? ›

Your pilot jet (or slow jet) controls the idle circuit, or from zero to one-quarter throttle opening. The pilot jet and airscrew control the amount of fuel and air going into the engine at slow engine speeds.

Does needle position effect idle? ›

Does needle position affect idle? The needle position doesn't directly affect the idle speed. With that said, in extreme cases it may have some effect. If you need to adjust the idle speed, you need to adjust the air screw (2 stroke) or fuel screw (4 stroke) and the pilot jet circuit first.

Does backfire mean rich or lean? ›

Lean Air/Fuel Mixture

Not only can a rich air/fuel ratio cause a backfire, a mixture that doesn't have enough gasoline can cause a backfire, too. A "lean" mixture is one that doesn't have enough fuel, and too much air.

Is high idle lean or rich? ›

The most common symptoms of lean jetting in a dirt bike are:

High idle. Hanging idle.

Is it better to run rich or lean? ›

Q: Is it better to run the too lean or rich engines? An engine running slightly rich will give more power, but running lean will cause catastrophic engine damage. Running too rich can also cause severe damage to your engine.

Do I need to Rejet motorcycle after installing new exhaust? ›

If you feel your bike is running correctly: It will not hurt your motorcycle to put on the exhaust and give it a test run. If you install the exhaust and experience poor throttle response, backfiring, popping on deceleration or excessive heat, then go ahead and rejet or add a Fuel Processor.

Is my 2 stroke rich or lean? ›

keeping the engine under a light load (you will only be opening the throttle to 1/2 or so) If the needle is set too rich you will hear a sputtering like the choke is on and the acceleration will be slow. If the clip position is too lean the engine will sound quiet and have a “dead” feeling possibly hesitating.

How do you adjust a running rich carburetor? ›

Regardless of whether or not the engine is running too rich or too lean, bring it down to a very lean mixture by turning both screws a quarter-turn at a time, counter-clockwise, then slowly bringing them back up to an equal and smooth mixture.

What causes a carbureted engine to not idle? ›

Often, dirt in the idle circuit can cause a no-idle situation, where the engine will run well at road speed. Sometimes, removing the idle mixture screw, blowing compressed air into the hole, replacing the mixture screw and readjusting the mixture may cure this problem.

Can you drill out carb jets? ›

By drilling out your stock main jet to larger sizes you will be able to narrow down what jet size your carburetor needs. You will need a numbered drill bit set. We purchased this set from Harbor Freight. The brass main jets are easy to drill so you can get by with one of these inexpensive drill bit sets.

How do I know what size carb jet I have? ›

What is the Size of my Jet? - YouTube

How do I know what size jet I need? ›

The suggested change in jet size is to go down one jet size for every 1,500 feet in density altitude you go up. Conversely, for every 1,500 feet you go down in density altitude, go up one jet size.

How do I know what size pilot jet I need? ›

The pilot jet is sized correctly if the fuel screw setting falls between 2.5-3 turns out. The pilot jet is sized correctly if the air screw setting falls between 1-2 turns out, with 1.5 being the sweet spot.

How do you figure out what cfm carb I need? ›

The formula for calculating how much CFM (cubic feet per minute) your engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456.
  1. Example: Using a 355 CID engine x 5,500 max rpm = 1,952,500.
  2. Take 1,952,500 x .85 = 1,659,625.
  3. Then 1,659,625 ÷ 3456 = 480 CFM.
2 Apr 2019

Do you jet up or down for altitude? ›

Higher elevations have less air density then lower ones. At high elevations our engines are getting less air, so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down 1 main jet size for every 1750 to 2000 feet of elevation you go up (info for Mikuni carbs).

How do I know if my carburetor is rich or lean? ›

Q: How Do You Tell if a Carburetor Is Rich or Lean? A: One way to tell for sure is by "reading" the spark plugs. If the plug tip is white, the mixture is lean. If it's brown or black, it's rich.

Do I need to Rejet motorcycle after installing new exhaust? ›

If you feel your bike is running correctly: It will not hurt your motorcycle to put on the exhaust and give it a test run. If you install the exhaust and experience poor throttle response, backfiring, popping on deceleration or excessive heat, then go ahead and rejet or add a Fuel Processor.

Is the pilot jet the same as idle jet? ›

An idle or pilot jet is a component of a carburetor.

It affects mixture at idle and just off idle. The main jet affects mixture at off idle all the way to wide open and takes over mixture of the AFR at higher engine/intake tract airflow speeds.

What does raising the needle jet do? ›

Raising or lowering of the needle just raises and lowers the sloping line, making fueling richer or leaner “on the needle,” which is from roughly 1/4 to 3/4 of throttle slide lift.

Can I drill out my main jet? ›

You should use a drill press for drilling your main jet. You will need a vice with jaws that will hold the main jet perpendicular to the drill bit. You do not want the drill bit to enter the main jet at an angle because it could create an oblong hole in your jet.

Which way do you turn a mixture screw to lean? ›

On most carburetors, turning the mixture screw in (clockwise) leans the mixture, while counterclockwise (out) enriches the mixture. Initially, if the engine stumbles or the vacuum drops when turning the mixture screw in, turn both screws out about a -turn and evaluate the results.

What happens if carburetor is too big? ›

If the barrels are too big, the loss of air velocity means the cylinder will not fill to its full capacity. An engine with a carb that is too big will put out less Torque and Horsepower . It will be difficult to drive due to poor low-end torque. If you drag race your car, an oversized carb will produce slow 60 ft.

How much horsepower will a 600 cfm carb support? ›

A 600 CFM carb can work well with engines making 250-450 horsepower depending on the combination of parts and the RPM where it makes peak power.

How much horsepower will a 850 cfm carb support? ›

6.1. Shown here with a Dominator, this Chevy big-block 572-ci engine proved to be capable of more than 750 hp in street trim with a 4150 carb (850 cfm, PN 0-80531).

Do engines run leaner or richer at altitude? ›

Higher Altitude Means Less Power

Reduced oxygen in high altitudes can lead to inefficient, sluggish engine performance because there is less air to feed the internal combustion engine.

Do I need to adjust carburetor at high altitude? ›

Power goes down with higher altitude.” For small changes in altitude, carburetors will compensate without any modifications. For larger altitude changes, carburetor jetting adjustments may be needed. A rough estimate is one to two jet size reduction for every 1,000-foot elevation increase.

Does High altitude affect horsepower? ›

As a general rule, a naturally aspirated combustion engine will lose 3% of its power for every 1,000 ft of elevation gain. If you have 100 horsepower at sea level by the time you get to 5,000 feet of elevation your engine is making 85 horsepower. At 10,000 feet of elevation your engine will make 70 horsepower.

Videos

1. Today I install a ProCycle jet kit on a Suzuki DR650.
(Dino's Tinker Shed)
2. Predator 212cc Jet Kit & E-Tube Install
(OMB Warehouse)
3. Bixpy Jet DIY Kit For Kayaks: Easy Motor Install
(KayakDIY)
4. JDJetting Carb Kit Installation MX Carb
(Johnnysshop)
5. Dynojet Kit Install
(Seacoast Harley-Davidson)
6. Motorcycle Jet Kit Detailed Breakdown with Patrick Daniels of DynoJet
(Cruiser Customizing Video)

Top Articles

You might also like

Latest Posts

Article information

Author: Rev. Porsche Oberbrunner

Last Updated: 08/27/2022

Views: 6061

Rating: 4.2 / 5 (53 voted)

Reviews: 92% of readers found this page helpful

Author information

Name: Rev. Porsche Oberbrunner

Birthday: 1994-06-25

Address: Suite 153 582 Lubowitz Walks, Port Alfredoborough, IN 72879-2838

Phone: +128413562823324

Job: IT Strategist

Hobby: Video gaming, Basketball, Web surfing, Book restoration, Jogging, Shooting, Fishing

Introduction: My name is Rev. Porsche Oberbrunner, I am a zany, graceful, talented, witty, determined, shiny, enchanting person who loves writing and wants to share my knowledge and understanding with you.