Why Constant Velocity Carburetors?
Most KZ and Z owners are looking for ways to bump up the performance of their bikes.
If you keep an eye on aftermarket carburetor options, you quickly discern that older VM29 and VM33 smoothbore carburetors and modern Mikuni RA Flatslides cost dearly and the prices continue to rise.
A less expensive and perhaps just as effective option is the use of refurbished Constant Velocity carburetors. Both Keihin and Mikuni manufactured this variety of carburetor.
Things to look for when purchasing a constant velocity type assembly is that it has a choke lever or pull knob on the assembly as opposed to use of a remote, cable operated choke lever.
A remote choke lever or knob means fabrication. It is also important to get an assembly with vacuum spigots on each carburetor so they can be synchronized easily.
Some assemblies DO NOT have this feature and rely on intake manifold-located fittings. Our seat-of-the-pants testing has indicated that a properly tuned set of CV carburetors will perform as well or outperform a set of smoothbores on a stock bike with only intake and exhaust modifications.
Which Carburetors to Use?
As noted, there are choices. The Keihin CVK34 or CV34 are used on KZ750 models. A drawback of the Keihin CV series is that some parts are unavailable (slow jets in particular). The Mikuni BS32/34 family has better parts availability. wiredgeorge Motorcycle Carburetors recommends and sells the more readily available BS34 assembly which works great in almost all applications.
Installation Tips/Steps
First, remove your old carburetors, airbox and carburetor holders. The picture below shows the old airbox and intake manifolds. Note that we use stainless socket head bolts (6mm w/1.0 pitch 16mm long) to retain the intake manifolds.
Install New Intake Manifolds
Install the new intake manifolds. You can see that the ones for the CV carburetors are MUCH larger in diameter. These are the intake manifolds used for 1981-1983 KZ1000J model carburetors. They bolt right up after elongating the mount holes with a file or Dremel. Note that there is no vacuum spigot on these manifolds.
Prepare the Intake Manifolds
After the J model intake manifolds are bolted to the cylinder head, install clamps and lubricate the inner surface of the manifolds with WD40. Do this EVERY TIME you install carburetors and they will slide into place easily!
The BS carburetor uses a single pull cable with different ends than are used on a VM style throttle cable. The following steps will show you how to quickly change out and adjust the cable. Cable adjustment can be done when the carburetors are installed. The VM cable screws into the right hand switch gear and is held by a locknut on one end and the other end has a threaded barrel with the cable going through and two locknuts to hold it on the throttle linkage. The BS cable also threads in the hole in the right hand switchgear and the other end is smaller to fit into a cup fitting on the throttle linkage of the BS assembly.
Cabling Steps
Remove the two M5 screws that hold the right hand switch gear together and in place.
Remove the locknut and unscrew the cable end from the switch gear.
Remove the cable end from the throttle inside the switchgear. Remove the old cable from the throttle sleeve and note its routing.
The new cable goes into the front opening of the switch gear. Screw it into place and tighten the 14mm nut. Turn the throttle so that the cable holding sleeve is pointed upwards and put the new cable end into the front slot in the sleeve and the cable will wrap around the front of the throttle. Reattach the two switch gear halves with 5mm screws over the throttle.
Route the throttle cable back to where it will be joined to the carburetor assembly. Don’t put any kinks in the cable as you route it to the rear and it would be a good time to lube the cable with some cable lube.
Pick up the carburetor assembly and put the cable end in the small hole in the throttle butterfly and then pull the cable sheath to expose bare cable and put the end in the throttle holder cup.
In the image below, the cable has been inserted into the cup. Now, go up to the switch gear housing and at the cable end, loosen the locknut on the cable adjuster and turn the cable adjuster counter clock-wise so that there is slack in the cable as shown. A BIT SLACK IS GOOD! The primary reason for high idle speed after a carburetor installation, is not enough cable slack!
Slide Carburetors In Place and Tighten Clamps
Slide the assembly into carburetor holders and hold it tight as you tighten the clamps. Note the rubber vacuum cap covering the vacuum port used for synchronization. If these vacuum ports are not connected to either the gas tank petcock via a vacuum line, or the emissions related system, they MUST be capped!
Fuel Hose and Pod Air Filter Installation
The illustration below compares a pod used for a VM application (26-28mm - on right) to a pod used for the BS34mm series installation. Both pod types are manufactured by Emgo. Install and tighten the clamps. Put your fuel hose on the fuel inlet (between carburetors 2 and 3) and use a hose clamp to secure it. Use 1/4" or 5/16" ID fuel line and an inline filter.
Fuel Hose w/Filter and New Pods Installed
This image shows a Mikuni BS34 carburetor assembly installed with pod air filters and fuel line. You are now ready to fit your gas tank and connect the gas hose to the petcock.
The gas tank is fitted and the fuel line connected. The installation is complete. Allow wiredgeorge Motorcycle Carburetors to help you upgrade the performance of your motorcycle with the installation of a Mikuni BS34 kit! Details are available on our Carburetor Sales page of this site.
