Jet Needles / Main Jets / Gas Mileage

My '82 KZ650 had stock main jets and single groove jet needles. The bike got great gas mileage (52 - 57 MPG), ran smoothly, but seemed to lack power from 1/4 throttle and above.

I used washers to raise the jet needles by 4 mm., and increased the Main jets by two sizes. Now the bike runs really strong, but the gas mileage dropped to about 43 MPG.

From what I read, if I go very easy on the throttle, (never open more than 1/4), then the Main jets should not affect gas mileage. Is this true?

I expect that I may have gone a little too far on raising the jet needles. Does anybody know the spacing between the grooves on adjustable jet needles? How many grooves does the 4 mm. equate to? Please advise. Thanks.

If I remember right, the needle slots are 1mm apart. So you've raised them 4 notches. That's quite a jump.

And that would be more than what would be on a needle with notches. Be careful they don't leave the needle jet bore and stick your slide valve open at WOT!!!!
Sounds like you are masking over another problem, such as vacuum leaks or possibly a low float level, leaking slide diaphragm, or some PO has done some drilling on the air bleeds or changed gets/ needle to something leaner.

Carb needle shims are shown in the Sudco catalog (Mikuni Tuning Manual 4th Edition):

i.d. .106
o.d. .250
thickness .020 (.5mm)

Seem to remember reading somewhere that 2 shims equals one clip position, which is consistent with Skyman's recollection of 1mm between clip positions.

Good Fortune!

The main jets will not affect your millage at all, unless you ride at wide open throttle all the time the needles will. One to two mm is a big change on a needle. Four mm will definitely kill your millage.

WABBMW wrote:

I expect that I may have gone a little too far on raising the jet needles. Does anybody know the spacing between the grooves on adjustable jet needles? How many grooves does the 4 mm. equate to? Please advise. Thanks.

On mine the grooves are about .030" apart. I think that is 0.75mm?

Is the .75mm reading center to center or just the thickness of material between the slots??

bountyhunter wrote:

...On mine the grooves are about .030" apart. I think that is 0.75mm?

NO FAIR measuring! :laugh: :laugh:

I plan on changing the jet needle position again, and from what you provided, I should be able to dial in just right this time. The stock setting seemed quite lean, so I may go the equivalent of one or two grooves richer than stock, instead of the quantum leap that I did.

I'll provide feedback as to how it works out. Thanks everybody.

I am happy with 35 miles a gallon I usually ride hard so fuel mileage is out the window

They should be 1mm (.040") apart center to center.

Becker wrote:

Is the .75mm reading center to center or just the thickness of material between the slots??

Center to center across the slots. I used dig calipers across five slots measuring from the top edge of the upper and lower slot, then divided by four. I think that's the right value, but my eyes are not as young as they used to be. I couldn't bet my life that it's not 1mm.