Tuning the Posa Carb

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 Oklahoma about 8 years ago, you won't have enough mixture adjustment left. Therefore the only answer I have heard and made work is to reshape the needle. Now comes the hard part, as if this message isn't too long already. To adjust the needle you need to make two measurements with a vernier caliper or a micrometer. These will tell you how much to build up your needle. First, go to full throttle with the airplane tied to something on the ground and your mixture control at full rich. Then cut off the fuel to the engine and the engine will speed up since it is running rich. When the engine has stopped, shut of all mags and electric and get out of the plane without changing the throttle setting. Get access to the needle to the Posa from the front of the plane and put a vertical scrape on the needle. (#1) Next, get back in the plane and again start the engine (back to idle) and go to full throttle. This time lean the mixture out to the correct mixture according to either the EGT or RPM whichever method you have available. Again without changing any throttle or mixture setting, shut off the fuel and let the engine die. This time the engine shouldn't gain in RPM much at all. Shut everything off without moving the throttle or mixture controls and go around and make another vertical scrape on the needle. (#2)  Finally, set the throttle (you can do this visually) with the air slide in the center. ( This is the point you had it running with the correct mixture without playing with the mixture) Make another vertical scrape on the needle. (#3) At this time you may remove the needle. Note: I counted the turns as I removed it to ensure when I put it back in I put the needle back in the exact same place. I did this by unscrewing one turn at a time and pulling on the needle with pliers until it came out.

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

                                                      adjusting screw


        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.

 If you chose to do the idle end, (I did) all you have to do is repeat the process on the other end. GOOD LUCK. Perhaps this is too detailed for only words and I could be talked in to removing my needle at the next Gathering and with a few pictures I think I could make the explanation more understandable.

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.

Jim Faughn

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.