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.?
