Reshaping the AeroCarb Needle

  40 Hours worth of flight testing and a study of slide carb geometry lead me to the conclusion that the #2 needle I was using in my AeroCarb was not shaped optimally. Flight testing showed that the carb was running rich at low power, and slightly lean at high power. The fuel metering needle used by the AeroCarb is a cylindrical brass rod, with a taper cut or ground into it. The tapered rod (needle) moves with the slide, and as the slide moves out, so does the needle. Because of the taper, more fuel is allowed to flow into the engine because less needle is in the way blocking it. The diagram below should help explain this.

The actual temperatures change on every flight, but a generalized Exhaust Gas Temperature (EGT) trend was developed and added to the diagram, as well as the mixture setting, in terms of needle turns. All references to rich or lean were to the starting mixture setting, which was right on at about 3/4 throttle. This produced the following needle profile:

With this data, we know if the taper of the needle is too much (i.e. allowing too much fuel), or not enough (too lean). Knowing how many turns we need to make in order to adjust the mixture will allow us to estimate the amount of needle we should remove. In order to calculate this, you'll need the slope of the taper. In my case, it was 0.034. This is a dimentionless number, but can also be thought of as 0.034 mm or taper per mm of length. Measure the length in which the needle is advanced for each turn (thread pitch of the adjustment screw), and we can calculate the amount of taper for one turn. In my case, this was 0.05 mm.

Lastly, we can do the math and convert turns rich/lean to mm of material needed to remove from the needle. For example, if we're 3/4 turn lean at WOT, then 0.75 turns * 0.05 mm/turn = 0.0375 mm removed. So, shave off approximately 0.03 mm at the exact point where WOT is reached (in my case, station 29.4 mm), and the needle will allow the proper amount of fuel to pass.


So much for the theory...

The reality of making this work is much more complicated. Every change, no matter how small, will affect the performance of the needle. You have to work very hard to control these second and third order effects, or you'll spend way too much time chasing the mixture. I know this from experience. I re-tapered 3 needles. Each one had its own mistakes, and the frustration level was pretty high.

From my experience, I'll offer a few points.

  1. Make very small adjustments. I recommend only cutting off a maximum of one half of the correction you think you'll need, and then trying it out. A little bit goes a long way, and if you over-cut a needle, you'll need to replace it and try again.
  2. Changes to the surface finish of the needle (even minor changes) will have a noticeable effect on mixture. If you want to polish out the surface imperfections, do that first, then run the needle to determine the rich and lean spots. If you attempt to polish and cut, you're asking for trouble.
  3. Use fine-grit, wet-or-dry sandpaper (400, or so) to remove needle material. Make sure you don't acidentally round off any square corners. Don't make any grooves that run along the length of the needle (another second/third order effect). They'll change the flow, too. (You should be getting the idea about second and third order effects by now.)
  4. Under no circumstances should you modify the very beginning of the needle (i.e. the idle end of the needle). It's tempting to want to smooth out the machining on the taper. The needle even has a bit of a step where the taper begins. Don't touch this area, or in fact, the first 1/3 inch (i.e. Station 0 to Station 10). Even a few swipes from fine sandpaper will mess up the idle performance, and it'll be nearly impossible to correct. Once modified, you're better off tossing the needle and starting over!
  5. Make sure you put marks on the needle that will allow you to re-install the needle to as-close-as-possible to the same place you took it out. This will make re-tuning the carb a ton easier after removing the needle to re-taper it (a little at a time means you'll need several iterations!).
  6. Don't modify your only needle. Order some spares to work on. That way if things go to hell, you can always put back in the original needle.
  7. Be prepared for some frustration. The stock needle may not be perfect, and individual needles may exhibit differences compared to one another, but they give pretty good service in a very demanding role. Considering your skill, determination, and/or luck, you may be better off leaving the needle alone!


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Updated: 19 May 08