jet needles

[twocycle]

Does anyone know how to calculate the percentage of fuel delivery
change by raising or lowering a Mikuni jet needle one clip? The carb
is VM34.

Thanks

 

[NO HAND]

The way the needle works, it mostly affect from 1/4 to 3/4 throttle opening. To make it leaner, you raise the clip towards the edge. You might want to take a look at the work of James Dean concerning needles. (Click Link)

 

[twocycle]

What I am looking for is a way to calculate percentages. For example the
difference between a 260 and 270 main jet would be be roughly 4%. Is there
a way to calculate percentages for needle clip positions?

Thanks

 

[RJH]

You could do that – I guess on the round slides – and it’s great to hear your interested. That model has many needle jets and jet needle combinations to choose from. The problem being - the opening size will change infinitely as the thing moves – so you really couldn’t get a reading. The needles all have different tapers and at different times. Your point of measurement is at the point where the needle leaves the jet needle. Another factor is the air correction jet changing the mixture slightly – if you change it. The only way would be to develop a computer program that would act like a sliding scale – changing percentage as you go.

You would have to take the ID, at the top, of the jet needle and subtract the OD of the needle jet - but where - as it is tapered. What we do is put several ink marks along the needles and mike them – that’s a good way to see what way your going and at what throttle position (another story).
You can’t take it at say only 3 positions, as the VM needles are not linier as some of the current TMX needles are. I can’t comment on the PWM type carbs.

It would be easy – but expensive to have the orifice space flowed at an instrument shop and calculate flow there –but once again changing other jets will affect the flow. The main is a specific size and it is quite common to determine %’s as you described.

If you measure everything and ride, on the needle, you will soon be able to feel the changes. Many advocate that it’s not important – but many don’t realize how little they are even on the main jet. Bob Hogg :cool:

 

[twocycle]

Thanks!


That is what I was looking for. As a general question is
one clip position a large or small change? I understand
that clip position will cause different amounts of leaness or
richness depending on throttle position.

Thanks

 

[twocycle]

I think I found my answer. On the Mikuni website. They show an approximation of clip position change. It looks like 3-6% at 1/4, 5-8% at half and 1-2% at 3/4 up.

Thanks for everyones help

 

[marcusgunby]

One clip change can be alot-it can go from too lean to gurgling rich-on the older crs i had to have a needle half a clip richer for winter use.

 

[twocycle]

Yes I found some .45 mm brass washers at a hardware store.
I think I will try a half step.