Mikuni VM series, round slide carburetors
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The Mikuni round slide carburetors used on many snowmachines are rather simple in design, but with parts which can be rather confusing in their terminology. The picture above shows just about any part which is made to be removable and has a name. This picture is not intended to help you in any way other than to identify parts.
- Disclaimer: It should be understood that the example used in this pictorial is a salvaged carburetor. This carb was not removed from a running machine - nor would a machine likely have run with it installed. It does show some of the issues one might encounter, albeit to a greater degree than one would expect to find normally. This carburetor actually shows very little wear in spite of the crud both inside and out. It will easily function perfectly again.
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.
To access the fuel inlet jet 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.
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.
The inlet needle sometimes has metal tip, other times a soft synthetic material ( ®). Since the needle and seat are subject to all the bums the machine has been exposed to, a metal-tipped needle should be inspected to make sure it is free from wear and grooves.
- If the inlet leaks for any reason, the engine will likely run in a flooded condition and, in some cases, the tank may even drain into the engine while it sits parked. A "stuck" engine that produces a geyser of gas when a spark plug is removed and the engine turned over can be a sign of a leaking inlet. (It could also result on the impulse side from a faulty fuel pump.)