Mikuni is a California-based manufacturer of carburettors especially popular for use in motorcycles and small engines -- particularly engines that use single carburettors. The Mikuni VM 26 SS is part of the Mikuni VM Round Slide line of carburettors, the most popular carburettor for single-cylinder motorcycles ever made, particularly for dirt bikes which often use single-cylinder, single-carb configurations. "SS" indicates it is a special edition of "Special Set" of the VM Roundslide.
The function of a carburettor is to mix fuel and air for combustion in the engine. This is done by introducing a fuel spray into air travelling into the engine. The air travels through a venturi, which is a large tube in the carburettor body. The throttle moves a slide that, when closed, stops the air/fuel mixture from flowing through the carburettor, and when open permits it to flow freely through the venturi. The diameter of the venturi is 26mm across. Mikuni VM SS carburettors come in a variety of sizes. The digits "26" in the VM 26 SS indicate its size.
VM Special Set
The defining feature of the Mikuni VM series is a round slide. While the VM Round Slides come automatically on many motorcycles, they may also be a performance upgrade on motorcycles with a different stock carburettor. The "SS" VM Roundslides are a special edition or "Special Set." They come with factory-specified jetting and throttle valve for a specific application. VM SS carbs, in addition to coming in a variety of sizes, also come with either a left-hand or right-hand idle screw. Other than having optional left/right idle screws and factory specified adjustments, the VM 26 SS is essentially the same as the VM 26 Standard model.
Fuel Circuits and Air Adjustments
The VM26 SS, like all carburettors, modulates air by opening and closing the slide. The rev range is the lowest rpm (idle) to the highest rpm, and corresponds to the throttle position. The throttle is closed at idle. The more the throttle is opened, the higher the RPM. A pilot jet and adjustable air screw affect the fuel/air mixture from idle until the throttle is about one-quarter opened. The throttle valve controls mixture from one-eighth to one-half throttle. The jet needle/needle jet system controls the mid-rev range from one-quarter to three-quarter throttle. The air jet and main jet control the mixture from three-quarters to full-throttle. The final adjustment is choke, which opens an additional air circuit to help the engine start when it's cold. The choke makes the fuel/air mixture leaner.