Mikuni needle jet and jet needle calculator

[alexbrown64]

A while ago i had trouble finding a specific jet needle that was recommended in a Yamaha Race Report.  Some needles are just not made anymore and its hard to find a similar needle as sometimes the profile specs are not given and you dont have an original etc.  Anyway, on another forum i gathered as much info as possible using Mikuni's own specs and measurements from members who had new jet needles that were not listed.  I then put together a few tables of needle jets and jet needles and made an excel spreadsheet calculator.  Table 1 gives jet needle diameter at 10mm increments.  Below that,  table 2 gives jet needle area as a cross sectional slice.  Chart 1 subtracts the area of a jet needle at a given point from the area of a selected needle jet.  Note: needle jets remain the same internal diameter for their entire length.
With this calculator, you can plug in your existing jet needle and needle jet and it will give you a value at every 10mm increment along the profile of the jet. The value being the space in square millimeters that is left between the needle jet and jet needle.  You can then plug in various jet needles and change needle jets to get differing values to suit your needs.  You can also see what the various jet needle profiles do at a specific point.  This is good to know if you have a bike that is perfect at 1/4 to 1/2 but you can never get right at 3/4.  Use the calculator to find a needle profile that works better at just that point.  I have put up a screen shot of the calculator and if you want a copy to have a play with then just email me and i will send you a copy.

 

Cheers,
Alex

[suzuki59]

That's great thanks a lot for sharing.

[Lozza]

I would hate to see all your good work go to waste, but what you need to consider is how it interacts with throttle position, clip position and the main jet. When the annulus of the needle/needle jet is bigger than the main jet it is just a discharge nozzle it is no longer metering fuel the main jet is. While the needle/needle jet annulus is smaller then it is still metering the fuel.