Topic: Mikuni Carb info from Z1enterprises.com

[Stev-o]

 Z1 KZ900 Carb Identification & Specification
 
  Click  link for pics...

https://web.archive.org/web/20120204062747/www.z1enterprises.com/reference/z1-carb-guide.aspx

 
Z1 & early Z1-A Mikuni 28mm carbs

The early Z1 28mm carburetors are best identified by the combination of 17mm drain plug directly under the main jet, and the angular top cover held on with just TWO Philips screws.

With the exception of the early bikes, this style of carbs was on all Z1's and all Z1-A's up frame number Z1F-32817.

On the early Z1's are very similar carbs, but there are a couple of subtle differences.  The carb body has less reinforcing ribbing - most noticeable in the smooth area in the main body, and very minor differences in the choke linkage.  These carbs seem to have been fitted all the way up to engines in the 3000 range (Z1F-03xxx).



Late Z1-A & Z1-B Mikuni 28mm carbs

Part way through the Z1-A production run (After Z1F-32817), Kawasaki modified the carbs to help address performance / tuning issues with the earlier bikes.

The modified carbs are identified by a small drain screw angled to the side of the float bowl.  Internally, the slide, needle jet and pilot jet were modified as well as the choke fuel pickup pipe placement.  Not only did the drain plug change, but because the choke fuel pickup point changed, the float bowls between the early and late versions are not interchangeable.  Externally, the carb linkages also changed.

In conjunction with the change of carbs, the ignition advance unit was changed as well - the advance unit previously ranged from 5-40 degrees of advance, and was changed to just 20-40 degrees of advance when paired with these later carbs.



KZ900 Mikuni 26mm carbs

With the release of the KZ900-A4 model in 1976, the carburetor design changed considerably.  The size was reduced to 26mm, the body of the carb was totally redesigned.  The top cover is more rounded and held on with THREE Philips screws.  The float bowl retained the small drain screw design from the Z1B style carbs.  Much of the design change was focused on smoothing out the raw power of the early Z1's into a more refined power delivery.  These carbs tend to stay in tune better given the improved design of the synchronization linkage.



Mikuni 29mm smoothbore carbs

Once power-hungry riders realized the potential of the Z1 engine, a large array of performance parts appeared on the scene.  One of the most important of these was the Mikuni 29mm smoothbore carbs.  The stock carbs did not adapt well to the increased fuel demands of high-lift cams and high-compression pistons, nor the back pressure changes of the barely flow restricted 4-1 exhaust systems.  The 29mm smoothbore carb was by-and-large a cure for many tuning problems.  The top of the carb is identical to the 26mm (and later KZ1000 28mm) carbs.  The float bowl went back to using the 17mm drain plug directly under the main jet.  Great for racers as it allowed for quick changes of the main jet.  None of the VM29's have an overflow pipe fitted in the float bowl.
lNote: some very early sets of 29mm smoothbore carbs have a float bowl with a small drain screw angled to the side.

Below is a reference of jet settings for the 29mm carb on Z1 / KZ engines.  This is a good starting point for a bike with 4-1 exhaust, after market cams (up to 0.410" lift) and high compression pistons (approx 10.5:1).  Each bike setup is different, so actual settings will vary.

We have included the Honda CB750 settings as a reference.  You will notice the similarity of setup - so.... if you can find a set of 29mm carbs off a CB750, they should work on a Z1.  The same is not so true with smoothbores setup for the Suzuki GS range.  The slide cutaway is different (2mm) - yes, you can still buy new slides, but they are expensive.


 28mm (VM28SC)    Z1 & early Z1-A
 Main Jet    112.5
 Air Jet    1.0
 Needle Jet    P-8
 Jet Needle    5J9-3 - last number is the clip position where 1 is at the top of the needle (leanest) and 5 is at the bottom (richest)
 Throttle valve (slide)    2.5mm cutaway
 Pilot Jet    20.0
 Air Screw setting    1 1/2 turns out
    
 28mm (VM28SC)   Late Z1-A & all Z1-B
 Main Jet    112.5
 Air Jet    1.0
 Needle Jet    O-8
 Jet Needle    5J9-2 (last number is the clip position)
 Throttle valve (slide)    1.5mm cutaway
 Pilot Jet    17.5
 Air Screw setting    1 1/4 turns out
    
 26mm (VM26SS)   KZ900A4 & A5
 Main Jet    115
 Air Jet    1.0
 Needle Jet    O-6
 Jet Needle    5DL31-3 (last number is the clip position)
 Throttle valve (slide)    1.5mm cutaway
 Pilot Jet    17.5
 Air Screw setting    1 3/8 turns out
    
 29mm smoothbores    Z1 / KZ900 / KZ1000
 Main Jet    120
 Air Jet    0.9
 Needle Jet    O-6
 Jet Needle    5DL31-3
 Throttle valve (slide)    1.5mm cutaway
 Pilot Jet    17.5 or 20.0 (or 25 in the later versions)
 Air Screw setting    1 1/2 turns out
    
 29mm smoothbores    CB750
 Main Jet    115
 Air Jet    0.9
 Needle Jet    O-6
 Jet Needle    5DL31-3
 Throttle valve (slide)    1.5mm cutaway
 Pilot Jet    17.5 or 20.0 (or 25 in the later versions)
 Air Screw setting    1 1/2 turns out
    
 29mm smoothbores    KZ650
 Main Jet    105
 Air Jet    0.9
 Needle Jet    O-4
 Jet Needle    5DL31-3
 Throttle valve (slide)    1.5mm cutaway
 Pilot Jet    15
 Air Screw setting    1 1/2 turns out
    
 

[Terry in Australia]

Interesting stuff Steve, when I finally assemble my 1975 Z1-B with it's Moriwaki 1100cc big bore kit, ported head, bigger valves, under-bucket shims, Norris cams, Dyna 2000 ignition etc, I'm running Mikuni 33mm smoothbores. No idea what the setup is for them, but what I've discovered with Mikuni carbs as opposed to Keihins, is that they're a lot less sensitive to minor adjustments, which suits a "bush mechanic" like me......

[BenelliSEI]

Steve..... thanks for all this! Really needed th info on my latest project, much appreciated.

[Jeff.Saunders]

The Z1 carbs were decent performing carbs - but not without issues.

The needle is not kept firmly in position in the slide - so it tends to get sucked towards the engine side of the needle jet - and rubs the needle jet hole into a visible oval (and wearing the coating off the needle too) - gradually richening up the midrange over time.  I do know of a place that has replacements though....

The 'dog bone' sync washers tying the carb pairs together easily bends - and it's tough to keep the carbs perfectly synced - esp. with the wear to the needle jet.

[MauiK3]

I discovered that my carbs have a larger thread on the bowl drain/jet covers. I ordered two spares, one of which I'll make into a clear tube checker, they have the 17mm hex but the thread is smaller. I'll keep looking.
I'm also going to check my needles etc. more closely now that I've read this. My bike came with more needles that are new, I'll have to look to see who made them.
What markings should be on them?

[Jeff.Saunders]

The original factory needles were 5J9's.

The aftermarket needles are decent quality.  The original jetting that came on those bikes was a touch on the rich side.  But with the switch from leaded fuel to unleaded fuel - and now to the ethanol laced fuel, the bikes run a little different.

If you plan on getting new 4-4 pipes, then rich is a good thing.  It helps keep the header pipes from discoloring.

The choke circuit often clogs up  - and it is difficult to clear the little brass tube without removing it from the body - and that is not easy to guarantee keeping the brass tube in one piece.

[craz1]

Hi Jeff, haven't heard form you for a while. I agree with Jeff about the pilot tube. You should take them out and clean them. They do come out with a gentle twist/wiggle/pull. You usually find a bunch of corrosion in the tube. I use a twist drill slightly smaller than the ID and by hand ream the inside. The orifice on top is pressed in and I use a small section of copper wire to probe. Find a piece of stranded wire,14gage and pull one strand from it. Works well. Everything should be ultrasonic cleaned. I use a kit from Z1E which has worked very well. It also gives you the needle jet which most kits don't have,P8. The early bikes takes a different needle jet which I believe this has.
https://www.z1enterprises.com/keyster-premium-nencho-carb-kit-z1-1973-1974.html?fits-motorcycle=Kawasaki%2FZ1%2Fall-models%2F1973
Your carbs should look like this when done.

[MauiK3]

My choke circuits were plugged up and my choke plungers were toast.
I got all of it cleaned out and the plungers went to Robert Meggitt in the UK for new rubber parts. They look great. I hope it all runs ok once I get it all back together.

[Don R]

 Thanks, this helped me ID my brothers 26's. and I learned a little about my 29 smoothbores.

[ScottJones]

Good morning. Scott here. I have a 1983 XJ550 Yamaha Maxim. I have a question about the Mikuni carbs that are on it. Why dosen't the manufacturer provide an over flow drain? It seems to me that if the floats are not adjusted correctly the carbs will overflow into the engine as the bowls are filling instead of draining out the overflow tubes as on my earlier Hondas. Any thoughts on this? Thanks.

[craz1]

Good morning. Scott here. I have a 1983 XJ550 Yamaha Maxim. I have a question about the Mikuni carbs that are on it. Why dosen't the manufacturer provide an over flow drain? It seems to me that if the floats are not adjusted correctly the carbs will overflow into the engine as the bowls are filling instead of draining out the overflow tubes as on my earlier Hondas. Any thoughts on this? Thanks.

I was wondering the same myself. I just completed a XJ550 restoration. Bottom line is you really need to make sure the float levels are correct and you have new float needles with the rubber tips.