Dr. Kayak's Pretty 1980 KZ750G
The first Kawasaki KZ750 twin was built in 1975 and sold as a 1976 model as a KZ750B1. For the first four years of production (1976-79), the KZ750B1 through B4 shared identical Mikuni BS38 carburetor assemblies. These assemblies are probably the heaviest twin-carb assemblies ever built and have a couple unique features. The biggest oddity is that the Kawasaki version of the BS38 uses a system where both the pilot jet and main jet are screwed into the float bowl. A good bowl gasket is critical because gas is drawn from the jets into the internal passages that lead to the venturi via channels beneath the gasket inside the float chamber. The pilot jets used are standard BS series fare in that they are Mikuni BS30/96 type but the main jets are unique to Kawasaki BS38 carburetor assemblies. They look like very small air jets and are frequently stripped as they require the correct sized small screwdriver to remove.
KZ750B1-B4 Carburetor Parts
In the picture showing the rebuild parts for a KZ750B note the odd float bowl gasket with all the holes in the middle. This is to accomodate the passages for the main and pilot jets inside the float bowl. The pilot mixture screw does not use an oring. The other gasket to the top-left of the picture is for the choke plunger sub-assembly. This assembly needs to be removed to get the plungers out and the term choke is a misnomer. What is called a choke is actually an enrichment circuit. The BS38 assembly doesn't have a choke. The pilot jet (Mikuni BS30/96 type) is the larger of the two jets shown in the bottom-left and the main jet is that itty-bitty thing to its right.
1976-79 BS38
1976-79 Mikuni BS38 front (1980 BS38 looks identical)
Specifications BS38 (updated 11/13/2014 due to errors)
| Mikuni BS38 1976-79 KZ750B | |
| Service Fuel Level | 4.5mm to 6.5mm |
| Main Air Jet | 1.0mm |
| Main Jet | 125 |
| Jet Needle | 4HL12-3 |
| Needle Jet | Z-2 |
| Pilot Jet | 40 |
| Air Screw | ~1 1/2 +/- 1/2 turns out |
| Float Valve | 2.0 mm |
| Starter Jet | 110 |
| throttle valve | 125 |
| 1980 KZ750G | |
| Service Fuel Level | 4.5mm to 6.5mm |
| Main Jet | 125 |
| Jet Needle | 4HL14 |
| Needle Jet | Y-3 |
| Pilot Jet | 40 |
| Starter Jet | 45 |
| Main Air Jet | 1mm |
| Throttle Valve | ? |
| Pilot Mixture Screw | ? |
The Kawasaki KZ750 Service manual specifies a non-US model set of specifications:(This info added 01/01/2015)
Mikuni BS38
Main jet 125
Needle jet Z-4
4JN19-4 (Y-22 aftermarket)
Pilot jet (also BS30/96 type) 45
Starter jet 55
1 1/2 +/- 1/2 turn pilot screw turns out
(Author's Note)
I have seen too many of these non-US sets coming through my shop and strongly suspect some were used on US models (1976-79). About half the sets I have seen are the non-US type and most have been sent in from US customers. Just a note on this. The Keyster carb kit pictured uses Y-22 jet needles which are closer to the 4JN19-4 profile than the shorter 4HL12-3 jet needles and the kit includes a #45 pilot jet. Keyster likely used the non-US BS38 assembly as a model for the kit. I have found that the parts interchange fine with both the KZ750B models and the KZ750G model. The Y-22 jet needle isn't a perfect match for the 4JN19-4 jet needle as it has a shade more taper and appears would be richer at the bottom end so perhaps clipping to the 3rd slot will work better to keep the lower rpm range from being too rich.
Note that the 1980 BS38 assemblies used on the KZ750B4 and KZ750G1 models look identical to the earlier assemblies so no pictures are included.
1976-79 Mikuni BS38 Rear View
BS38 connections
Each carburetor has its own fuel inlet. We recommend using an inline fuel filter. There are drain overflows and these should have drain hoses connected. The hoses should be routed over the swingarm. The carburetors in this assembly vent via the slow at the top of the venturi. In the event the bike overturns, gas will come out this vent so take care. Synchronization requires use of screw-in adapters to provide vacuum. There are slotted-head plugs on the side of each carburetor that remove and adapters must be screwed into the threaded holes for connection of a manometer or set of vacuum gauges.
The arm on the side of the assembly marked CHOKE is not actually a choke but actuates an enrichener circuit in each carburetor by lifting up plungers. These plungers have rubber pads that seal the circuit so that it doesn't flow any mixture when the "choke" is not pulled up. When starting a KZ750 twin, and the choke is pulled up, the enrichment plungers are pulled up out of their seats and extra gas from the float bowl is drawn into the mixing chamber and mixed with air supplied via a passageway below the slide diaphragm. This additional mixture mixes with the pilot (idle circuit) air/fuel to provide a richer mixture at start.
Enrichener Circuit Is Actuated When "Choke" Is Pulled Up
1982-83 KZ750 twin models
BS34 Carburetor Assembly from 1982 KZ750M1 CSR
1982-83 KZ750 Twins
For some reason, Kawasaki didn't sell a KZ750 twin model in the US in 1981. In 1982, they marketted the KZ750M1 CSR model and switched the carburetors from the BS38 assmebly that had been used on the KZ750B model family to the BS34 assembly shown above. The BS34 assembly weighs about 4 pounds while the BS38 weighs in at over 6 pounds. The BS34 assembly shares many of the internal and external parts used across the KZ line from the KZ750, GPz750, KZ1000P and all other KZ1000 models plus the KZ1100. I suspect that the move was made to standardize parts. Connecting these carburetors is fairly simple. There is no overflow system so no hoses need to be connected to the drain nipples. There are two tees between the carburetors in the picture. The lower tee is the fuel inlet. The upper tee is venting. This fitting should have a hose connected and run over the swingarm. In the event the bike overturns, gas will come out of the vent fitting and you don't want it wetting you if you are under the motorcycle. Note that the vent tee seems to have a brass fitting. This fitting was installed as a repair as the tee is made of all plastic and the nipple where the hose is to be connected was broken off. The brass piece in the picture is a repair.
BS34 Assembly Rear View from 1982 KZ750M1
BS34 Specifications
| 1982 KZ750M1/S/Y1 BS34 | |
| Service Fuel Level | 3mm +/- 1mm |
| Main Jet | 115 |
| Jet Needle | 5C50 |
| Needle Jet | Y-9 |
| Pilot Jet | 45 |
| Starter Jet | 70 |
| 1983 KZ750K1 Belt | |
| Service Fuel Level | 3mm +/- 1mm |
| Main Jet | 115 |
| Jet Needle | 5C50 |
| Needle Jet | Z-0 |
| Pilot Jet | 45 |
| Starter Jet | 70 |
| Pilot Air Jet | 1.6mm |
| Throttle Valve | 165 |
| Pilot Mixture Screw | 2 1/4 turns out |
BS34 Assembly Side View from 1982 KZ750M1