Mikuni VM Carburetor Connections
Overview
Most Mikuni VM series carburetor hoses can be connected properly if you understand why the hoses are needed.
Overflow Drains
Some VM series carburetors have bowl overflow tubes inside the bowls. These tubes allow the bowls to drain in the event there is too much gas in the float bowl. These tubes end in brass spigots on the bowl bottoms. A hose is necessary because in the event of an overflow, it provides routing for the gas away from the hot engine. The hoses typically will be gathered together with a wire tie and routed over the swingarm on the right side of the bike. Some VM series carburetors do not have overflow tubes and thus no overflow spigots. These are the assemblies with large plugs over the main jet (in the float bowl) for fast main jet changing as with early Z1/Z1A carburetors as well as VM29 smoothbores assemblies.
Bowl Vents
The carburetor bowls need ventilation in order that the float assemblies work properly. There are always two brass spigots on the sides of #2 and #4 carburetors for bowl venting. These vents connect to the #1 and #3 carburetors via short, rubber hoses to allow venting of those carburetors as well. These vents must not be obstructed or plugged. As with the overflow tubes, there should be hoses connected to the vents and these hoses run over the swingarm or in some vent into the OEM airbox on later models.
Fuel Inlet(s)
Fuel runs between the four carburetor bodies via much larger pipes. There may be two gas inlets or a single. The two fuel inlet type assembly will have inlets located between #1 and #2 and #3 and #4 carburetors respectively. These inlets are essentially "fuel tees". The single inlet assembly will have a single "tee" between #2 and #3 carburetor and straight pipes between #1 and #2 and also between #3 and #4.
Vacuum Fitting
A last connection may or may not be present. It is a vacuum fitting and looks exactly like the spigot for the vent. This fitting will be on either #2 or #3 carburetor body. If on #2 body, it will sit behind the vent spigot and if located on #3 carb body, it will be located as shown on the picture at the top of the page. This fitting is a passageway directly into the carburetor venturi. The purpose of this fitting is connect ion to a vacuum actuated petcock via a hose. I f your petcock has a "PRIME" position, it is a vacuum actuated type. If your petcock says "On/Off/Reserve", no vacuum connection is required. The vacuum spigot on a carburetor may also be connected to emissions control equipment where the vacuum actuates a valve that opens/shuts the reed valves in a valve cover designed for emissions control . In any event, this vacuum spigot MUST be connected to the device it was originally intended to operate with OR it must be capped with a vacuum plug. A 7/32" vacuum plug work best. If the vacuum spigot is allowed to remain open, it creates a major air leak and the engine may be damaged and the motorcycle will not idle properly.
This picture shows the location of a vacuum spigot. It is behind the vacuum spigot located on the center of the body of the #2 carburetor. It doesn’t show well in the picture but it is there. Sometimes a vacuum spigot is located on the #3 carburetor. It will be located on the flat and empty area where vent spigots are located on carburetors #2 and #4. See the top picture for an example of a #3 carburetor vent. The vacuum spigot in that picture is capped with a black 7/32” vacuum cap as it wasn’t needed for use on our test bike.