needle questions.

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the trail rider
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needle questions.

Post by the trail rider »

I know how the pilot and main jet are screwed in and such. but i need a explanation on the needle. what does moving the needle up one notch do and what does moving it down one do. and any jetting starting point is well appreciated. 220 with stock carb. opened up airbox, fmf gnarly desert pipe, turbine core II, v-force III reed valve, ride 500-5500 feet and usually temps from 40-80 degrees. thanks the trail rider.
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Post by Rhodester »

Here's my simplistic take on needle operation. You do understand that the engine acts like an air pump. The necked down venturi in the middle of the carb speeds the air up as it passes the center section of the carb creating a vacuum effect on small ports that are drilled in this area that lead to fuel in the bowl. The needle is like a partial plug that blocks this vacuum signal to the main jet. The needle is tapered so that as the slide (throttle valve) is raised, and the needle is lifted with it, it adds increasing amounts of fuel to the increasing amount of air that passes by the main jet port. It, in effect, unplugs the main jet gradually as needed. More properly said, it exposes the main jet progressively to the vacuum signal produced by the passing air in the venturi. The diameter of the upper part of the needle controls the flow of fuel into the air stream as the slide just starts to rise. Changing the diameter of this section of the needle will effect how rich or lean this part of the throttle opening is. As the slide continues to rise the tapered section then comes into play. To raise or lower the needle in this throttle area (by the clip position) will richen or lean the mixture in the middle throttle positions. You can also alter the pattern of enrichment by using a needle of a different taper angle or using one that is longer or shorter overall. The tip of the needle exposes the main jet to the greatest effect of the vacuum signal. To get a little more advanced, one must also deal with the fact that the engine is applying much more vacuum to the air downstream from the slide when the slide is closed, or nearly closed, as apposed to when it is fully, or nearly fully, open. The needle deals with very small changes in the fuel enrichment process, but can easily be felt. One clip position is a significant change. So there you go, FWIW.
'95 200H
the trail rider
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Post by the trail rider »

what jetting do you recomend and what needle position. so that I understand it clip position no 1 is at the top and clip position no 5 is at the bottom.
get a lift, fat chicks can't jump!

when in doubt gas it!
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Post by Rhodester »

I have no experience with the 33mm carb on the 220. If you had a 35mm or 36mm PWK carb that would be a different story.....sorry. You are right on the proper needle clip positions. Raising the clip lowers the needle and leans things up (and vise versa).
'95 200H
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Post by canyncarvr »

Number 1 is at the blunt end, #5 (or #6 sometimes) is on the pointy end. Yes you do understand correctly.

Hey! I can be agreeable, can't I?

re: 220 starting point?

First...a repeat..again, even. No one can jet by mail..or email for that matter. All bikes are different. Oils are different. They way you gapped your plug will make a difference...etc. etc....

Generally speaking an OEM 'H' 220 will be somewhere around 142/145 with a 42 pilot and the needle on clip-2. A pipe will add one to that main...make it 145/148.

IMO (THAT never works in jetting somebody else's bike either!) you will probably end up one lower on the pilot (a 40) and the Kaw needle is a waste of your time altogether. Less diameter, a later starting slope and a sharper slope (more º) will work better.

Here's a pivotal point on needles. Well...it was 'fer me!

L1 is the 'straight' spec of a needle. First off...it's not at all just the part of the needle that is straight! That's logical, ain't it? It is the length of the needle from the blunt (clip) end to the place on the needle where its diameter is 2.515mm. So...part of any L1 is going to include a tapered part of the needle.

Where the different slopes matter (well, one place anyway) is this: The more slope (steeper angle, more ºs), the sooner that 'magic number' (2.515) will be reached! Take an 'A' needle and a 'C' needle with the same L1 measurement: The 'C' needle taper will start further down the needle than will the 'A'. The faster the slope, the sooner you get to 2.515, so the later you start heading there!!

Aha! All of a sudden it is very evident WHY a different slope of needle matters in the lower throttle ranges, 'eh? That 'A' needle has a leaner taper, but it has a richer bottom end!!

Isn't that cool!

Of course it is!!! :wink:

An anecdotal aside..that's why the 'A' needles in your carb get you a loaded up plug on a long downhill (idle) run. The taper is out of the needle jet even with the throttle closed! You hit the gas after that downhill..and all you get is 4-stroke burble!!

A 'B' (OEM needle) isn't as bad, but it's close! That's part of the, 'Help me!! I keep fouling plugs!' whine 'ya hear all the time on this and other boards.

One more slight aside....the vacuum created by a venturi doesn't suck the fuel out of the carb bowl. Atmospheric pressure pushes the fuel to the lower pressure. Small matter...but it does matter!

Good write-up on needles and stuff, Rhodester! You KNOW it was simplistic...'cuz it made sense to me!! :wink:

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the trail rider
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Post by the trail rider »

hey that is a pretty good write up and it is understandable. I know the stock needle is not the best but it will work for a while for me. later on I think I will want something like a DEK needle that I have read so much about.
get a lift, fat chicks can't jump!

when in doubt gas it!
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Post by canyncarvr »

Oh no!!! :roll: The DEK is for girliemen!!!

A REAL man runs a 'C' needle!!!


...where's my blanky........ :neutral:

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Post by skipro3 »

Well said Carvr. You take the words out of my mouth!!!

The blanky comes with every KDX sold by your dealer. If you get a CEK needle however, they come with an iron jockstrap to support the large ones the buyer obvously has. :butthead: HaW!!!
Jerry

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Post by canyncarvr »

...or to firmly cover up what he don't....

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