Mikuni main jet needle info

JPaganel · Aug 11, 2012

The following is courtesy of Chacal. I did not see this in a search or in the catalog, so I PMd him and he replied with this.

I should note that the numbers he is talking about are stamped at the very top of the needle. You have to take it out of the piston to see them.

MIKUNI MAIN JET NEEDLES:

Attached to the vacuum piston assembly is the MAIN JET NEEDLE, and as the vacuum piston is raised or lowered due to the vacuum signal through the carb throat, the main jet needle is raised or lowered into the brass emulsion tube (also described as the "main jet" or "power valve" in some technical literature). These jet needles control the overall fuel flow in the main jet circuit of each carb.

Although many of these precisely made aluminum needles are not currently available, part of your rebuild efforts should be to verify that the correct size needles are installed in your carbs, and that all of the needles are the same size. The needle "size" is inscribed on the shaft of the needle, and normally takes a 3- to 6-character (letters and numbers) designation. Although most Mikuni needles are interchangeable between all types of the Mikuni BS-series carbs, their different "sizes" are designed to be used in specific applications as determined by factory design engineers.

Original main needle size configurations are as follows:

4GN: used on all 1981-82 XJ550 Maxim carbs.

4GZ1: used on all North American XJ550 Seca carbs.

4GZ3: used on all non-North American XJ550 carbs.

4CP3: used on all 1984 XJ550 Maxim on the #1, #3, and #4 carbs.

4CP7: used on all 1984 XJ550 Maxim on the #2 carb.

4DPS39: used on all XJ650 Turbo model carbs.

4HZ22: used on all XJ900RK carbs. This is a non-adjustable needle.

4HZ26: used on all XJ900RL carbs. This is an adjustable-position needle, and can be used on the XJ900RK models in place of the original 4HZ22 needle above. It offers far greater fine-tuning capabilities, as well as having a slightly "richer" profile than the 4HZ22 needle.

5FZ62: used on all XJ900 N and FN carbs.

5FZ10: used on all 900F carbs.

5FZ82: used on all XJ700-X and XJ750-X carbs.

5GLZ: used on all XJ1100 carbs.

A complete Mikuni main jet "needle" is actually composed of a few component pieces:

- the aluminum needle (shaft only).
- the needle "ring cap", which is the plastic "head" of the needle.
- the ring cap retainer c-clip....retains the ring cap to the needle shaft.
- the needle tension spring seat metal or plastic "spacer washer".
- a needle tension spring, to keep the needle tensioned in its bore. NOTE: this is not the large vacuum piston spring, but a very small spring that fits directly under the diaphram spring seat plate, and over the top head (or "ring") of the needle).

The above components, once assembled together, become a "needle" in Mikuni-speak. The needle is placed into the recess of the vacuum piston, with a small thin alignment pin on the bottom of the needle "ring cap" providing proper alignment of the needle in the vac piston. A flat metal plate, secured by two very small screws, fits over the entire list of above components, and holds everything in place. This flat metal plate is the spring "seat" for both the small needle tension spring (below it), and for the large vacuum piston spring (above it).
Click to expand...

bigfitz52 · Aug 12, 2012

A diagram of the differences between say, the 4GN, 4GZ1 and 4GZ3 needles would be a wonderful thing, wouldn't it?

Mikuni has a strong presence in the US, and an extensive racing program, etc.

I'm gonna ask them and see what they say. Stay tuned.

edfive55 · Aug 13, 2012

sorry for the dumb question but was the 85 and 86 xj700 using a totally different designed carbs. I have a hard time finding specs and what not on them

EskimoJoe · Aug 13, 2012

It is my understanding that the xj700 air-heads use the Hitachi carbs. The Mikuni carbs are only on the xj700x or water-heads.

JPaganel · Aug 13, 2012

EskimoJoe said:

The Mikuni carbs are only on the xj700x or water-heads.
Click to expand...

Also on the 550, which is what I have.

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Mikuni main jet needle info

JPaganel Well-Known Member

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