TOPIC:

I'm going through a set of early VM26's and two of the 4 Needle jet's are damaged.  They are 0-5 size.  Does anyone know what the -5 size indicates?  The parts fishe on Partzilla suggests that Kawasaki used from 0-3 to 0-5, depending on the model.  I've got some different VM26's with some 0-4 needle jets so what's the difference between the 0-5's and what running issues, if any, could be expected?

Thanks in advance for any feedback.

The needle jet works along with the needle taper.  For any given (mid-range, 1/4 to 3/4) throttle position and needle clip position, a larger needle jet will provide a larger opening (gap between the needs jet and jet needle), and allow more fuel to flow.  O-0 is more lean, O-8 is more rich.  I believe the '78 Z1R used O-1.  Sudco only sells them in even sizes, so 0,2,4,6,8.  My guess is going up or down one size would be a very small change.  I'd probably go up one size, as everyone says the late 70s carbs were pretty lean.

I scanned and attached a page from the Sudco Mikuni tuning manual.  The chart at the bottom left shows the actual size of the needle jet.



 

Here is that table, in photo form.  

Also, here is a graph from the Mikuni tuning manual that shows the change in fuel as the needle jets step up or down.

 

I've also attached the PDF of that tuning manual.
 

'76 version used O6 jets.  "77 and '78 used O5.  All 3 years used the same 5DL31 needles in the 3rd position in at least one model but '77 used a different needle in the A model.  If you have O4 jets they are probably from '79 or '80 years 28 mm carbs.  They would probably work with the needles lifted a position (or maybe two) to compensate for the tighter fit.

There should be a second (3 digit) number on the jets.  This indicates the series used at that time.  You should try to match that series.

Took one 0-5 and two 0-4 needle jets to work today with the intention of honing out the 0-4's to match the 0-5.  Used certified gauge pins to check the orifice size and found them all extremely close.  Went back to the chart posted here by DOHC and realized that the difference between them is only 2 ten thousands of an inch.  One of the 0-4's was virtually identical in size to the 0-5 and the other was ever so slightly smaller.  Pretty sure the large one is showing just a touch of wear.  I can't see the difference affecting mixture enough to matter.

That's what I thought when I said moving one step would be a very small change, but after my first post I looked at the graph.  I was surprised that they show a 5-8% change at 25% throttle for O-0 to O-2.  So maybe 2-4% for 4 to 5?  Small, but still surprising for 0.0002".

I read all about this when I was rebuilding my Z1R carbs.  I was trying to decide if I should try to replace the needle jets.  Once I found the diameter chart, it was clear that I wasn't going to be able to measure them to confirm they were in spec.  Of course, the O-1 isn't actually available in any case.  In the end I left them.  I just looked for wear on the needles, and didn't find much.  I figured if the needles are ok, the jets are probably fine?

By the way, jetsrus.com lists the O-4 and O-6 for $20.50 if you want to throw money at the problem.

Hard pass at $20 each!

The carbs in question are another set I'm rebuilding to flip so not looking to spend tons.  Some dumbhead pushed on the top of the jet to remove them from the carb body and caved in the cup shape on one side (on two jets.)
The worn 0-4 is super close to the 0-5.  I'm going to remove the last two 0-4's from my parts set and measure them then pick the best two.  

Even carbs from back in the day were jetted a smig lean, so if the jets are worn a tiny bit I'm calling that an improvement.