This was an e-mail I sent a number of years ago to the KR List Serve. I will try to update this and get some pictures of the needle to illustrate better now that I have a web site.....
I wish I could send a
drawing of the finished needle because that would explain it better than I can
with simply words, but I will try. The Posa has a major design flaw since the
air intake is a non-liner opening (a circle) and the fuel needle is linear
(Ground or cut at a constant angle. Small at idle and large at full throttle.)
The problem is that you will only be able to get the right fuel to air mixture
at one setting of the so called carb. Therefore, you must change the needle to a
non-linear shape to match the air intake. (This is easier than making a square
out of a round hole.) The way I did this was, with the mixture control set full
rich, set the air slide opening in the middle and adjust the needle for its
optimum mixture setting. At this
point you have the air and fuel mixture balanced at this throttle setting. Next
you go to idle and you will be running rich. At idle you have two choices.
First, you can change the adjustment on the carb to allow more air in through
the small hole in the slide( which is exposed at low RPM's)
which was Posa's attempt to fix the problem, or you can build up the
needle at that end with solder and scrape it so the fuel to air mixture is
correct. (I like to have it a little rich at idle in case I have to do an
emergency go around and when I push for maximum throttle I have a little extra
fuel to make up for the sudden inflow of air. Kind of like a pump circuit in a
carburetor.) The same situation will exist at full throttle except Posa did not
come up with a solution that I am aware of. . When you go to full throttle, the
air from the maximum setting is not as much as the amount of fuel you will get
from the needle. So, you will be rich. You might be able to adjust this out with
the mixture setting, however, if you go to 10,500 ft as we did at the KR
Gathering to meet the guys from
Now you have three scrapes on the needle. One in about the center, another near the end and another between.
Thinnest Thick where it
Attached to the
1 2 3
Measure with a Vernier the thickness at point 1 and at point 2. What you have to do is to build up the needle so it is the #2 thickness (less) at the #1 position. (Back to air fuel mixture theory - We are reducing the fuel that flows in since the air slide is not linear and didn't allow enough air in at full throttle to support the fuel.) Obviously this isn't the only point it has to be built up, instead it must be built up from point 3 all the way back to point 1 with point 1 ending up as the thickness of point 2. (If you aren't confused by now then God help us all). The way I did this was to get out my soldering gun and solder up the needle with a thin coat of solder just to the left of #3 building up to a thicker coat at point #1. Since you have the vernier measurements you can measure the thickness at point #1 and keep building up the solder until you get it right. Then after I built it up high enough I began scraping with a razor knife the solder to re taper the needle. This allowed me to change the thickness of the needle and ultimately get the right air to fuel mixture. The advantage of using solder is if you mess up you can do it again. What you are trying to do is make a non-linear needle for fuel flow to match the curve of the non-linear air slide.
The bottom line was that I am VERY happy with my Posa. I may be the only one in the world. My plugs burn a nice ash color and I have only changed them once in 170 hrs and I didn't have to then. Also, I would like to give special thanks to both Steve Bennett and Dan Diehl who got me on this track of re-tapering the needle. I changed their procedure but they helped me understand what was wrong.
Now that I have 250 hours on the engine, I have changed the plugs again but it was because I went to a new ignition and putting in new plugs seemed to be a good idea.