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Pilot needle performance 4 years 7 months ago #24910

  • scotch
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I'm separating this from the ongoing "lean Idle" topic to avoid bogging-down that thread.
In that thread, Kawboy commented: <I also have to say Scotch, your microscopic inspections and determinations of issues related to float needles and pilot jets is fascinating to me. It's Fluid Dynamics on a micro level and you have taken us all here at KZ1300.com to a whole new level of understanding of why the carbs on these bikes are soooooo finicky...>
This might be laughable to some but it's really no joke. It wasn't until recently having several sets of carbs on the bench that were claimed to be un-tunable, that I "clued in". Hence the Microscopic forensics. New pilot needles and/or new pilot (idle) jets cured the problems. I decided to play with a damaged set of pilot needles and made some interesting discoveries all related to fluid-flow and turbulence.
The photos will save a lot of words. The needle on the right is stock (damaged) The tips on these are machined squared-off. The modified needle on the left has been was re-profiled - shorter to remove the damaged surface material, and pointed......which was the objective of this experiment.

A better look with the new profile cleaned-up.

The theory:

The results were: Now shorter and with a steeper angle the modified needle has a narrower range of adjustment, relative to a stock needle. 2 turns + or - 1/8 turn. An unexpected benefit was with regard to "Color Tuning" The range of the "Blue" is now less then 3/4 turn, making using the Color Tune far more definitive in finding the "middle" of the Blue range.
There are 2 noticeable "improvements". The starting procedure (with the Choke Air Mod) changed significantly. Once the idle-fuel circuits have been primed, the choke can be reduced significantly - to a point where there is no plunger lift and the "roller" is barely on the fast-idle cam. More significantly, the COLD idle is notably smoother, will do so at 500 RPM and with little if any clutch/gear/chain noise. I attribute all of this to the "pointed" end of the modified needle and the smoother ( laminar flow) of fuel/air to the idle port in the venturi.
I've had these modified pilot needles in for about 2 months and they're staying in!

One final comment: Manual states: Pilot-needles = 1.75 turns. Reality =: 3 - 3.5 ish. The first production bikes were probably rolling off the line when at at some point the pilot-needle profile was changed (3.5 turns) but the manuals were already in print (1.75 turns) and no one noticed or thought to supplement the info.?
1980 KZ 1300 sr# KZT30A-009997
Always High - Know Fear !
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Last edit: by scotch.

Pilot needle performance 4 years 7 months ago #24916

  • strate6
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Interesting post indeed !

I have always removed all mixture screws on all my 1300's across the years and inspected the tips as soon as I own them..........not in as much close detail as you, but with a small magnifying glass. Also normally horrified at what people have set them at !!!!

Most I have found to be distorted at the tip (as you show), which I always assume is mainly due to previous owners in the past screwing them all the way IN, to the point where the tip collides with the hole in the Venturi, before setting tham at what they THINK is correct..............which is not good.
Depending on the extent of the damage, I either replace or carefully repair them.
Also I have found the only true way to set the correct "burn" is with a coloutune plug, which is the only real way to do it and agree that most settings are around the 3 to 3.5 turns OUT and nowhere near the 1.75 the K Manual states.

Pete F
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Last edit: by strate6.
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