Kawasaki Z1, Z1A and Z1B Carburetors
Pete's Z1B at historic NJ site
Pete Bartelli's pretty Z1B at George Washington's winter encampment, Jockey Hollow in Morristown New Jersey, the year after their terrible winter at Valley Forge.
Kawasaki chose to use a Mikuni VM28SC carburetor assembly on its Z1 model which was first built in 1972 and the first US models titled in 1973. This carburetor assembly uses external slide actuation mechanisms which adjust the height of the mechanical slides. Since four cylinder engines were a relatively new innovation, it appears Mikuni borrowed many of the basic design features of the assembly; especially the slide actuation, from earlier Keihin four cylinder carburetors. The Z1 assembly is very similar to the assembly used on the Honda CB350F, 1972-74. An exploded diagram of the CB350F Keihin assembly is included at the end of this article for comparison. Note the similar slide actuation parts used in both the Z1 and CB350F assemblies.
A “feature” of the VM28SC carburetors used from the beginning of product until the end of 1974 was the two return springs used to return the slides. This “feature” makes for a very heavy throttle pull compared to the 1975 version of the VM28SC or later VM carburetor assemblies. The early VM28SC also had asymmetrical floats which were not used on any other type of VM carburetor.
1973-74 Z1, Z1A VM28SC
The 1973-74 Z1, Z1A Mikuni VM28SC early style carburetor assembly was used on Kawasaki Z1 motorcycles produced in late 1972 through the end of 1974. The earliest versions of this carburetor assembly have some unusual features; no vertical strengthening rib, different enrichener (choke) actuators and different enrichener plungers and some other small features.
Early Ribless VM28SC Assembly Above
Early Choke Plunger
Early Enrichener (choke) Plunger Used on 72-74 VM28SC Versions
Later Early VM28SC Assemblies 1974-74
The later 1973 and 1974 versions of the VM28SC added the vertical rib, probably for strengthening, and while most 1973 assemblies are very similar, in 1974, Mikuni used a variety of slightly different parts throughout the production year. The parts used were probably some of the newer small parts under development and stuff they had lying around making 1974 carburetors tending to be slightly different with regard to small parts. The “CHOKE” arm was soldered to the actuator rod on the 72/73 assemblies but some time in 1974, Mikuni began using a separate “CHOKE” arm retained by a bendable washer and nut. The earlier choke arm has red paint-fill and a small plastic cladding on the lever end. The later choke arm doesn’t have the word “CHOKE” etched on the side but on its larger plastic cladding which is usually cracked in several places which necessitate repair.
ID 1470 or ID 1473 73-74 VM28SC
Later 1973-74 VM28 Assembly with Vertical Rib Above
The Z1/Z1A version of the VM28SC has external synchronization adjusters. These adjusters are tied together in a fairly crude manner. The basic mechanism was copied by Mikuni from earlier Honda Keihin four cylinder carburetors used on the 1972-74 CB350F. These external synchronization mechanisms have many parts and rely on cleanliness and good spring tension to function correctly and have a reputation for not staying synchronized indefinitely like later internal synchronizing mechanism assemblies.
The 1972-74 early VM28SC used a unique pilot jet; Mikuni type VM28/213 in size #20. The early VM28SC assemblies are the only ones to use this pilot jet.
VM28/213 Mikuni Type Pilot Jet
Early Choke Arm
Note the red paint fill on the choke arm. This is a sign of an earlier production VM28SC (1972 or 1973). The assembly ID mark is located on the carburetor flange directly below the paint fill in this picture. It is a 4 digit code in the form 147x where X indicates one of several possible assemblies.Three assemblies were used from the start of production through the end of 1974. The rarest was the no-vertical rib type. This type carried the ID mark 1470 but the next version of the VM28SC also carried the 1470 ID mark and these were used through 1973. Another assembly with ID mark 1472 was used during the end of 1973 (along with the 1470) and into early 1974 when the ID mark for the Z1A changed to 1473.
Fixed synchronizer on carb #4
As mentioned previously, the VM28SC assembly uses external syncrhonizing mechanisms. The 1973-74 versions used number four synchronizer in a fixed positioni by capping it with a hex fitting so it would become stationary and the other three carburetors would need to be syncrhonized relative to the number four carburetor. In 1975, this changed and the number four carburetor was given a normal synchronizing mechanism.
Early VM28SC Uses Set Screw and Locknut
The 1973-74 VM28SC assembly uses a set screw and locknut to retain the choke plunger actuator arm to the choke arm shaft rather than a normal round head set screw used in later VM carburetor assemblies starting in 1975.
Z1A VM28SC
1974 Z1A VM28SC Assembly ID Mark 1473 with Overflow Barbs
1974 VM28SC Z1A Assembly
Not much changed from the 1973 Z1 VM28SC assembly to the 1974 except overflow tubes were added. Overflow tubes catch fuel that is above their top level and route it through the tube and out the barb external to the bowl in the event that too much gas fills the bowl. It is intended that the barbs have hoses attached and routed over the swingarm on the right side of the bike. In the event the bike overturns or a float malfunctions, extra gas is routed away from the engine via the hoses.
The above picture is a 1974 VM28SC version of the VM28SC with ID mark 1473. Not much different looking than the 1470/1472 assemblies.
1972-74 Z1/Z1A Mikuni VM28SC Specifications
| ID Marks Used | 1470, 1472, 1473 |
| Service Fuel Level | 3.5 mm |
| Float Level | 24 mm |
| Pilot Air Jet | 1.0 mm |
| Pilot Outlet | 1.2 mm |
| Main Jet | 112.5 (a few early models used 125) |
| Jet Needle | 5J9-3 |
| Needle Jet | 243 P-0 |
| Throttle Valve Cutaway | 2.5 |
| Pilot Jet | 20 |
| Air Screw | ~1 1/2 turns out |
| Float Valve | 2.5mm |
| Starter Jet | 40 |
1975 VM28SC Z1B Mikuni Carburetor Assembly
In 1975, Kawasaki had Mikuni update the VM28SC carburetor assembly mainly to improve the motorcycle’s idle. The slide cut-out called the Throttle Valve Cutaway in documentation was reduced from 2.5 to 1.5. The slide cutaway is the dished-type cut on the bottom of the slide that allows air through the venture when the slide is closed. A larger cutaway area decreases air speed through the venture and necessitates a larger pilot jet. In 1975, this cutaway was decreased and a smaller pilot jet was used. In addition, the pilot jet was slightly redesigned, probably to give more strength to the thinnest part that fits into the pilot jet well. Changes in the timing advance curve were also made to help achieve a smoother idle as the mechanical advance was redesigned with a different timing curve. The mid-range for the 1975 assembly was leaned a bit by using a smaller needle jet and re-clipping the jet needle one slot higher. My guess is that these changes were made to compliment the slide cutaway changes. Suzuki went through much the same process on their GS750 models where they also started with a very large slide cutaway and then transition to a much smaller cutaway with smaller pilot jets used.
Kaw Service Bulletin 74 Z-31
1975 Z1B VM28SC Specifications
| ID Marks Used | 2170 |
| Service Fuel Level | 3.5 mm |
| Float Level | 24 mm |
| Pilot Air Jet | 1.0 mm |
| Pilot Outlet | 1.2 mm |
| Main Jet | 112.5 |
| Jet Needle | 5J9-2 |
| Needle Jet | 243 O-8 |
| Throttle Valve Cutaway | 1.5 |
| Pilot Jet | 17.5 |
| Air Screw | ~1 1/2 turns out |
| Float Valve | 2.0 mm |
| Starter Jet | 80 |
1975 Z1B VM28SC Carburetor Assembly Above
For the 1975 VM28SC assembly version, Mikuni used a one piece throttle return spring which made throttle turn effort somewhat easier. The float bowls lost the characteristic large hex plugs and used 6mm side oriented drain plugs. The slide cut out was decreased, speeding up air flow through the venture at idle so a smaller pilot jet could be used and a better idle achieved. The mid-range of the 1975 carburetor was leaned a little by decreasing the size of the needle jet 10 full sizes from P-8 to O-8. The 1975 VM28SC assembly switched pilot jet types to the pilot jet that is used on most VM assemblies subsequently (on the larger displacement round slide assemblies). This type pilot jet is Mikuni type VM28/486. The 1975 assembly used a #17.5 size.
Mikuni VM28/486 Type Pilot Jet Above
1972-74 Keihin CB350F Carburetor Assembly Above
1973 Z1 Exploded Carburetor Drawing
There is a strange similarity between the CB350F carb assembly of the day and the Z1 assembly. Makes you wonder... Cheers! wg
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