Needles

[karmarepair]

AeroCarb needles: I'm currently running a 2.5 on the advice of a noted fellow flyer (Jeff Schultz), but my engine (early production but low time, AeroCarb) is running rich at idle and lean at higher throttle. Would the original #2 needle better match the "slope" of the fuel demand as the throttle opens? Naturally aspirated, at sea level, about 60 degrees today, measuring AFR with a gauge,running 100ll with a bunch of Decalin.

[Bryan Cotton]

I started off with a #2 needle. Got it dialed in and was really happy with it. Then I had to screw with things and I added fresh air induction. There is a whole thread about it, including my experiments:
viewtopic.php?f=7&t=4465

After fresh air induction, and before I got the mixture balance between cylinders figured out, I tried a #2.5 needle. It was way too rich all the time, except for WOT. I went back to the #2 and was happier.

But then I messed around some more with induction/balance and really got it running great. Except now that it is cold, it's way too rich at idle and cruise and too lean at WOT.

So Brock made this awesome picture explaining the difference:


You can find that here and other places:
viewtopic.php?f=33&t=1791&p=52567

I recently ordered a couple more #2 needles. I'm going to tweak mine leaner, mark the spot at WOT where the needle is just entering the fuel orifice, and then do some tapering from just below to just above that WOT spot. Somebody else did some custom needle tapering, but I can't find the thread. The two extra needles are in case I screw up.

I also run 100LL with decalin and I measure EGT at every cylinder. #1 is the hottest for me but after all the screwing around with my intake manifolds all the other ones are within 120F of #1. Not quite as good as the VeeCU but I'm pretty happy with that level of balance.

Also for reference my intake mods can be found here in this thread:
viewtopic.php?f=7&t=3232&start=130

[karmarepair]

Tapering needles: http://users.lmi.net/~ryoung/Sonerai/Fi ... _Carb.html
Jeff Shultz tried it, gave it up, and went back to stock tapered needles, in his case (Jabiru 3300 currently) a #3. http://www.sonex604.com/carb_tuning.html

[Onex107]

The pictures of the needles may not be perfectly accurate but the spots you chose for equal width have different thicknesses. In my needle studies I determined the thickness to be the determining flow control. As you show, the angle of the needle taper increases with number. I found the wot hot spot was at .o82 thickness and the idle hot spot at the beginning of the taper. The object is to get these two spots within the slide movement. The #3 needle puts them closer together than the others due to the increased angle. If you have to lean at idle, after setting the best wot, they are not close enough together. For my Aerovee, the #3 gave me the correct mixture setting for both throttle positions. I only changed the mixture at cruise to raise the EGT. No change for landing and taxi. I also changed the ball socket in the needle holder that had .025 of play, and also check the diameter of the needles. Mine were under .125 by several thousand ths. I made new needles out of .125 drill rod.

[Bryan Cotton]

The pictures of the needles may not be perfectly accurate but the spots you chose for equal width have different thicknesses.

If I remember from his original post, the pictures were exaggerated so the differences could be seen by eye easily. You must have been the guy who made his own needles - sorry I could not remember who. That's pretty cool.

[Onex107]

I did make seven new needles out of .125 drill rod, gradually changing the angle, increasing it, from #2. You can calculate the angle by measuring two spots an inch apart and doing the math. The angles of the needles I had made were set up on a Sine plate on a surface grinder and were very accurate and reproduceable. After a lot of interesting adjusting and testing, my needle #7 worked perfectly, but, when I compared #7 to the Aerovee #3 I found they were almost identical. There was a builder in Florida who told me #3 was the answer to my throttle problem but I didn't believe him. The common thought about the needles is that the higher the number the richer the needle is. Not true. The two hot spots you need to attain are just closer together in the slide distance.