Every carburetor know to man will have a way to adjust the air fuel ratio at idle. This adjustment will trim the mixture so that you can have a smooth running engine.
They work opposite of each other so it is important to know that before performing this procedure. What you are after when adjusting this screw, is too reach peak idle RPM and smoothness. This is the engine telling you it is happy, and it runs best at this mixture.
The pilot jet is sized correctly if the air screw setting falls between turns out, with 1. A pilot air screw will be made of brass, gold in color, and will have a blunt tip compared to a pilot fuel screw.
I use the smallest diameter screwdriver so I generate the least amount of torque. A pilot air screw will be located on the air box side, while a fuel screw will be located on the engine side. This is true for most carburetors.
When in doubt, remove the screw and see what kind of tip it has, that will determine what it meters. This video walks you through removing them.
Now, adjust between these two settings to achieve highest RPM and smoothest running engine. Reset idle to recommended settings and test ride motorcycle. If they dip below idle, lean the mix. When you close the throttle and decelerate, the engine runs lean, and can pop through the exhaust.
Sometimes this is considered normal with a race engine and open pipe.
Hi, this website is very helpful for me. Thanks before. But i have an issue on my mikuni bs. Its fit while engine on idle. But when run test, the engine feel rich and the spark plug is black.The following tips and insights are for your information only. These will not make you an expert tuner. We recommend that if you are a novice, you consult a professional mechanic or technician for your specific needs. The author cannot be responsible for your interpretation and usage of this information.
To visualize how the various circuits overlap, please refer to the jetting chart. Always remember to change one carburetor component at a time and keep a record of your changes.
Pilot air jet / Keihin FCR (MX) carburetor
Depending on your type of riding, adjust the minimum idle speed to desired RPM making sure the engine is up to operating temperature. If you do not desire any idle, make sure you turn in the adjusting screw just enough so the engine will not idle.
This is especially important on Keihin PJ Series carbs in that the idle adjust knob 4 in illustration cannot be completely closed. Such an adjustment will result in a sluggish response off idle. The carburetor pictured in the exploded view uses an air adjustment screw 5 in illustration that is located "upstream" of the throttle valve slide and meters air. Turning the air screw counter-clockwise leans the mixture off idle. Some carburetors have this screw located "downstream" of the throttle valve, in which case, the screw meters fuel and opening the screw counter-clockwise results in a richer mixture.
The idle screw usually has a range of one to two turns out from fully closed. Consult your owner's manual for the standard setting. The slow jet calibrates the mixture from both the idle bypass and the idle orifice in the jet block.
Consequently, if the throttle is only partially opened, such as in a trailing throttle situation, and the bike tends to load up, emitting a deep tone when the throttle is returned to full open, it is usually a sign of a rich slow jet. If the slow jet does not clean up this part of the circuit, the slide can be substituted for one with a different cutaway.
How it overlaps with the jet needle depends on the jet orifice inner diameter, air bleed holes and type of nozzle screen. MAIN JET: The best track side method to determine the size of the main jet 7 in illustration is to fully load the engine on a long straightaway or hill. At the end of the stretch, chop the throttle and hit the kill button simultaneously. Now pull the spark plug. Best power will usually result in a very light tan colored insulator tip and dark colored ring around the tip of the electrode.Genuine Keihin jets provide the best replacement or calibration solution for your Keihin carburetors.
Do not use an imitation, nothing substitutes for OEM quality and Keihin jetting is guaranteed to be the exact size indicated because of high manufacturing tolerances. They replace the ZF5 old slow air jet and allow you to fine tune to an infinite numbers of positions without replacing any jets!
Just follow the chart in the details page and you can tune it to any jet size you wish. The kit contains a screw and a spring, the jets have a 7mm thread diameter. You remove your fixed size ZF5 slow air jet and install the screw with the spring installed over the end. See the chart below for the jet size to adjustment correlations. We promise to never spam you, and just use your email address to identify you as a valid customer.
After havimg bad running symptoms on my drz, i bought this to rejet the carb. What a saviour! Bike is running like a dream now thanks to powerbarn! Thanks alot!
All prices are in USD. Please wait Sign in or Create an account. See 1 more picture. Buy in bulk and save. Product Description Genuine Keihin jets provide the best replacement or calibration solution for your Keihin carburetors. Please verify fitment before purchasing. Customers also viewed. Add to cart. Newsletter signup Name Email.This is the base guideline about tuning your Keihin. Certain parameters may vary due to air pressure and temperature, altitude, umidity. Be carefoul when jetting, a bad jetting can produce huge engine failures.
Don't do it yourself if you don't feel confident but let it to a professional mechanic. This section features the PWK exploded views just to let you understand where the tuning components are located.
The guidelines are the same for both 2 and 4 stroke. On FCR carbs the main jet, slow jet, jet needle, air adjustment, slow jet air adjustment and gas valve are different and located elsewhere.
The slow jet calibrates the mixture from both the idle bypass and idle orifice in the jet block. If the slow jet does not clean up this part of the circuit the slide can be substituted for one with a different cutaway. The higher the last two numbers the leaner the needle the lower the letter the richer the needle. By going to a thinner needle there is a larger area between the jet needle and needle jet supplying a richer mixture.
If you have to run your clip in the highest position a needle with a longer straight section will need to be used. If the engine is too rich above a quarter throttle raising the needle clip 1 in illus. For example, a 1. The number of tapers is not usually changed. How it overlaps with the jet needle depends on the needle jet orifice i. It is also not advisable to calculate how rich or lean a needle jet is using exclusively the nozzle inside diameter to needle outside diameter discharge area.
Wide open throttle w. At the end of the stretch chop the throttle and hit the kill button simultaneously.
Keihin FCR and FCR-MX Carburetor Adjustable Slow Air Jet Conversion Kit
The plug is then pulled for a "reading". Best power will usually result in a very light tan colored insulator tip and dark colored ring around the tip of the electrode. The electrode itself should have fairly sharp edges. For example, if the ceramic insulator has a nice tan coloring but the electrode has a white ring around the tip and the plug is of the correct heat range then you can easily run a size larger main jet.
Also, that race or av gas is more prone to oxidization and storage deterioration, along with the fact that a multitude of types are used worldwide. When jetting your main jet try to remember to jet for the best power for a given track.
An example of this is endurance where you would want to run a main large enough to keep the engine cool, this means that you may be on the rich side but the engine will fade less towards the end of the race. Another situation could be a stadium sx track where you spend much more time on the low to mid circuits.
As you gain experience and knowledge, you will be able to use other methods to determine your jetting. A good tuner can "feel" most of the circuits by slowly revving a parked bike, or just by looking at the color of the unpainted pipe and silencer.
Tips This is the base guideline about tuning your Keihin. Zero throttle operation - idle circuit idle screw: depending on your type of riding adjust the minimum idle speed to desired rpm making sure the engine is up to operating temperature If you do not desire any idle make sure you turn in the adjusting screw just enough so the engine will not idle.The air screw is a small 5mm in diameter slotted brass adjustment screw located on the inlet side air filter side of the carburetor.
The airscrew is a fine-tuning adjustment designed to allow the carburetor to be slightly adjusted for variances in atmospheric conditions. Proper adjustment of the airscrew can offer direct feedback on the necessary setting required for the pilot jet. The airscrew is adjusted in a rather straightforward manner. The ideal procedure for setting the screw in the correct position is to warm up your scooter engine to the proper operating temperature.Pilot Air/Fuel Screw Adjustment Explained - Multiple Carbs - Part 2
Continue backing the airscrew out until the engine idles at its highest RPM. The preferred setting window is between 1 and 2 turns. If the engine idles at its highest RPM from turns out this means the pilot setting is on the Lean side and a larger pilot jet should be installed. If the engine idles at its highest RPM at over 2 turns out, this means the pilot setting is on the Rich side and a smaller pilot jet should be installed.
Clean the system thoroughly with contact cleaner and blow out with compressed air. Carburetor must be disassembled. With proper air screw adjustment and a close initial setting from your engine tuner, fine-tuning should be painless. Once set the pilot jet is not terribly sensitive. You should only be required to adjust the setting when confronted with large weather changes or altitude swings of over ft.
If adjusting the pilot jet gives inconsistent feedback, or does unexplainable things. The slide not only monitors how much airflow goes into your engine its main job. This is referred to as slide cut away.
The slide cut away is measured in 4. The higher the number, the larger the cut away the leaner the slide setting is. The slide cut away is usually predetermined by the engine manufacture or engine tuner. As a general rule do not change the slide cut away unless instructed to do so by a skilled engine tuner.
The jet needle is the most important component in determining your carburetors jetting. The needle is broken into 3 main functions; Diameter, Length, Taper. In the following paragraphs I will explain the needles functions and how to adjust them. The setting of the needle diameter is crucial to both the engines low RPM power and reliability.
On gold colored needles identified with the 3 stamped in letter I. In many instances you can leave the taper and length settings the same if they are correct and adjust only the diameter. When the needle diameter is Lean the scooter will have a loss of low-end power. The engine will feel very zingy.
When an engine is in this condition and then put under a heavy load the engine becomes very susceptible to seizing. In extreme cases the engine can feel like the choke is on or the plug is fouling.
When the needle diameter jetting is set correctly the engine will accelerate evenly thru the first part of the power band.If you don't see the Information you need E-Mail Us:. Keihin Main Jets. Keihin Main Jet 9mm in length with a 8mm Round Head. Keihin Press In Main Jet 9.
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