Float Bowl Parts

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Mikuni internal parts a.jpg


Float Bowl

Mikuni Main thrubowl.jpg

To access and maintain any internal parts one must remove the four screws retaining the float bowl on the bottom of the carb. (Here, the sediment trapping tube has been removed revealing the main jet. The main jet can be replaced as necessary without removing the bowl. It does require a special tool.)

Removing the float bowl may require a solid work surface of some sort - even if it is only improvised. Having a properly fitted Phillips screwdriver (#2 tip) and adequate pressure is important to prevent the screw heads from being stripped.

Mikuni details.1.jpg

With the bowl removed, major internal parts become visible. They are identified here.

Fuel Inlet

Mikuni float.arm.jpg

To access the fuel inlet seat and needle, remove the float arms by pushing out the pin. Some carbs use a headed pin which only goes in and out one way. Others (like this one) use a headless smooth rod.It is held in place by the cast shape of the bowl when it is installed.)

Mikuni inlet.jpg

With the float arms removed, the needle will simply drop out if the carb is turned over (to the normal operating position). The jet or seat can be removed with a 10mm wrench or socket. Watch for, and don't lose, the gasket.

Mikuni inlet needle.jpg

The inlet needle sometimes has a metal tip, other times a soft synthetic material. Since the needle and seat are subject to all the bumps the machine has been exposed to, there is going to be wear. A metal-tipped needle should be inspected to make sure it is free from wear and grooves.


Jet: a fuel port with a hole of a specific size designed to deliver fuel at certain rate. Fuel flows because the air pressure on one side of the jet is greater than on the other. Normally, fuel will flow from a jet and mix with high speed, low pressure air flowing past.

Main Jet

Mikuni main.jet rate.jpg

The main jet, just like every other jet and needle, is marked to identify it. This main jet is marked with the number 145 which indicates that 145 CCs of fuel will flow through it in one minute under certain conditions. The main jet, along with a brass washer, secure the needle jet in body of the carb. (Keep track of that washer and make sure it is re-installed when the main jet is installed or the needle jet will not be secured.)

Pilot Jet

Mikuni jet ports.jpg

The pilot jet is fairly hidden inside a deep recess. It is important for idle and low speed operation and should be maintained.

Needle Jet

Mikuni jet.marks.jpg

The needle jet (left) and pilot jet are both marked to indicate their specific characteristics. (A digital camera can actually be helpful in reading the very tiny marks impressed into the jets.)

Maintenance and Repair

Mikuni float arm level.jpg

With the inlet seat and needle, and float arms installed, the float arms should hang parallel to the gasket surface of the carb body when it is held upside-down. To adjust the arms, bend the tab which bears against the needle.

Mikuni pilot needle.jet.jpg

An engine which does not run properly may have issues related to debris or corrosion. A good filter should prevent unwanted solids from getting to the carb. Water, however, especially saltwater, can have very negative effects if it is allowed to sit for long. Periodic (annual) inspections are never a bad thing. These jets both have soft white "stuff" trapped in places that will interfere with fuel and air flow. This type of material is a result of moisture interacting with non-ferrous metals. The easiest way to avoid it is to keep water out of the fuel by fueling from a clean source and avoiding condensation.

It is important to note that not all machines come equipped with in-line fuel filters, nor is access, if they have them, easily possible. This is especially true of some of the newer machines which have so many parts so compressed into small spaces. Consequently, it is even more important to keep moisture out of the fuel system. This can be a challenge when one considers the fact that many machines get fueled from secondary containers in this part of the world.

To avoid moisture related fuel and carburetor problems, always fuel from a clean source (or don't completely drain the container from which you are fueling) and keep the machine's tank topped up to avoid condensation. Fuel additives such as isopropyl alcohol can be useful in removing moisture that accumulates - and some always does- over time.

Return to Main page Mikuni VM series, round slide carburetors

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