Mikuni main jet size chart

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Mikuni Jet Identification Chart

Mikuni Motorcycle Carburetor Theory Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch PSI. This means that atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow. Atmospheric pressure will force high pressure to low pressure. As the piston on a two stroke engine goes up or goes down on a four stroke enginea low pressure is formed inside the crankcase above the piston on a four stroke. This low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and engine until the pressure is equalized. The moving air going through the carburetor will pick up fuel and mix with the air. Inside a carburetor is a venturi, fig 1. The venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carb. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor. Most motorcycle carburetor circuits are governed by throttle position and not by engine speed. There are five main metering systems inside most motorcycle carburetors.

How to Understand Carburetor Jet Sizing


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.

Jet Size Chart


So the dealer gave me some jets an they say RD on the side but mikuni only list 's then 's. Jets look fairly close to all the others I have. Are they from a different manufacturer? So Mikuni jets go from tono Like This Unlike djm 14 Jan So the dealer gave me some jets an they say RD on the side but mikuni only list 's then 's. Don't know what the RD stands for, should be an N after the number They started using these in 01 and 02 on the 's and 's. The following was posted by "driggs" on another thread, good info. What happened was Polaris engineers discovered that a jet actually flowed less than a The jet flowed grams of fuel per minute while the jet only flowed grams of fuel per minute. It was discovered that Mikuni uses a different numbering system when it comes to hex jets larger than the size. The was an accidental overlap in the two numbering systems. To remedy the situation Mikuni assigned new jet numbers with an N suffix. The following is a chart showing jet size, equivalents, and fuel flow. Main Jet Fuel Flow grams per minute The 's had as I recall 's in 00, 's in 01 and the N's in These carbs did not have hooded needle jets, and therefore did not pull enough gas in the midrange due to low signal strength. They went to the larger and larger jets due to warranty issues on engines burning up in the mid range. Then in 03, doo went to the hooded needle jets in the flat slides on most engines and the hoods caused more pressure drop across the needle jet, pulling more midrange fuel. When they went to the hooded needle jets they dropped the main jets to 's in general. I can't get into the doo parts catolog right now or ronnies fiche either to confirm the numbers I don't disagree with the N jets being equivalent to a main. And there were a lot of us running 's in our 01's and they came with straight 's. It was weird as the 01 's came with a N jet which is equivalent to a regular jet as I recall, jet the 's had the straight But if you get the needles you can run a or main with no issue. Get much better fuel mileage and run much stronger on top end.

Mikuni - Keihin - Dynojet Conversion Chart


The first thing we do is match the stock main jet to its corresponding numbered drill bit. Search Engines. Drilling Main Jets Deciding which main jets to buy to tune your carburetor can be a difficult task. You may not know what jet size would be a good starting point for your combination. Why buy a bunch of main jets based on a guess when you may not ever use them again? 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. Run the engine with the stock main jet and take a plug reading. If it's lean and you need to increase the size of your main jet, drill the jet out with the next size larger drill bit, which in this case is the 60 bit. Run the engine again with the drilled out main jet and take another plug reading. Repeat this process until you get a good plug reading. See our " Carburetor Tuning " section to learn more about plug readings. Do not skip drill bit sizes because once you drill the main jet too large, you cannot go back to a smaller size. 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. Once you drill the main jet to a size that gives you a good plug reading you can use the chart below to determine what size the jet is. In the photo to the left you can see the drill bit measures 1. This means we have drilled our stock main jet to the equivalent of a main jet. Deciding which main jets to buy to tune your carburetor can be a difficult task.

Carburetor help, jetting, and tuning help:

The jets on a carburetor meter the amount of fuel that enters the throttle bores of the carburetor where it mixes with incoming air. If the engine shows performance problems such as stalling or sluggish acceleration, you'll need to change the jet sizing. Jets are threaded and have a small orifice through their center that varies based on the size of the jet. For example, a jet may be stamped on its head with the number This indicates that the jet's size is 30 mm. Installing a smaller size jet will reduce fuel flow, which is ideal for high-altitude conditions where air is limited, where a larger jet size increases fuel flow for oxygen-rich, low altitudes. With this knowledge, you'll be able to understand how to correctly change the jets to regain the engine performance levels that were lost due to improper jetting. When the operating altitude of the engine will be feet higher than your normal altitude, you'll need to insert a jet one or two sizes smaller than the size currently installed in the carb, which reduces the amount of fuel entering the engine to match the reduced level of oxygen in the air. If the altitude is dropped feet, you'll need to insert a jet that is one to two sizes larger than the current size, increasing the fuel flow to match the higher oxygen levels. An incorrect ratio will show in decreased engine performance. Unscrew the fuel bowl bolts with a wrench and pull the fuel bowl off the carburetor. Save the gasket that was pressed between fuel bowl and the carburetor's metering block what the fuel bowl is bolted to. With the bowl off, the jet s are exposed and can be identified by their brass color and center orifice. Unscrew the jet s with a screwdriver or pair of pliers and note the size stamped on the jet's head. As mentioned in step 1, you'll need to increase the jet size for lower altitudes, while a smaller jet size will be needed for higher altitudes. Make the size change in increments of one to two sizes at a time. Screw the new jet size into the jet mounts within the metering block, with a screwdriver or pair of pliers. Press the fuel bowl gasket and fuel bowl against the metering block and tighten the bowl bolts with a wrench. Turn the engine on and test it by listening for a smooth idle and a crisp throttle response. If the carb continues to under perform, remove the bowl and make another one to two jet sizing adjustment, either up or down based on your circumstances, until the engine runs well. Install a smaller jet size if the ambient temperature dips below degrees Fahrenheit. This creates a lean condition that runs the engine hotter, ideal for cold conditions. If the air temp raises above degrees, larger jets will be needed to increase the amount of fuel entering the combustion chamber, known as a "rich" condition. Rich conditions allow the engine to operate at a cooler temp. Unbolt the fuel bowl bolts from the metering block of the carburetor. Pull the bowl away from the carburetor, preserving the bowl gasket. The jet s are now exposed and can be identified by their center orifice, screwed into the metering block. Unscrew the jet s with a screwdriver of pair of pliers and adjust the jet size up or down one size, depending on your circumstances. Screw the new jet s into the jet mount s in the metering block and press the fuel bowl gasket and bowl to the metering block. Tighten the bolts of the fuel bowl to complete the jet sizing change. Test the engine to ensure the jet sizing change has compensated for the temp change, indicated by the engine running smooth during idle and crisp under acceleration. Change the jets whenever you increase the performance characteristics of the engine. The could be anything from a larger intake manifold, long duration cam shaft or milling the cylinders for increased displacement. In essence, when an engine's performance level is increased, more air is required to run the engine. This ratio is different for each engine, requiring you to refer to your engine manual, new component's install manual, or by simply testing. Remove the carburetor bowl, bowl gasket and jet s as explained in the above sections. Replace the jets with a larger jet sizing by two to three sizes. Reinstall the jet sfuel bowl gasket and bowl as explained in above sections.

How to adjust a carburetor, jetting and mixture explained!



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