SonexBuilders.net

I posted this as an email to the sonex tech email. Thought I'd post here to see how many different responses I can get....


Background:
I have the 32mm AeroInjector and stock needle on my Revmaster R2300. I have it adjusted so that on climb-out (3200 rpm) I'm flowing about 6.6 gph and the engine feels to be running fine at this current setting WOT. A 1/4 turn leaner and the engine stumbles on take-off.

At cruise (2700-3000 rpm) I lean to about 4.5 gph. At idle I need to lean much more for a smooth idle.

Problem:
If I am at full mixture and pull back to idle the engine is too rich and sometimes dies (on ground). I don't dare go full rich and idle when on short final. However, to be prepared for a possible go-around I want to be able to do just that (full-rich/idle) while on short final.

Question:
Is there another needle, if so which one, that would lean out my idle rpms yet still allow the same fuel flow at WOT that I have now?

Thank you!
-brs

PS. Don't ask about EGT's as I've given up on getting meaningful data from my MGL probes and display. Could be I drilled the probes too far from the heads.

Are you sure the issue is “too rich” at idle?

Have you looked at your fuel flow when you pull back to idle and noticed whether it ever goes to zero or near-zero?

And do you have a gascolator or un-insulated fuel lines in the engine bay?

Because a far more common issue in the Sonex is “the burps”, which are vapor bubbles in the fuel line that can cause the engine to stall at idle or near-idle throttle settings during ground-runs or when landing after the engine bay gets hot.

Everything connected to the fuel system (the carb, the fuel lines, a fuel flow sender, etc) all conduct heat back into the fuel system when they get hot.

—Noel

What is your idle RPM?

XenosN42 wrote:What is your idle RPM?

Ha, I thought (after hitting post) someone would ask that. With the over-rich idle mixture, as I have, obviously it's too low else it would not die. I think its in the range of 700 RPM, however when I agressively ground lean, it goes up to something line 1300 RPM. Today I'll warm it up and get the exact numbers.

However, I feel that bumping up the RPM to fix an overly rich mixture is a 'FIX' that should be considered after doing what is possible to have good fuel metering. It might come to that. But then the side-effect is possible fouled plugs unless I lean at idle. This bring me back to...

My Aerovee will idle less than 500 when it is warm. It doesn't sound happy but doesn't quit. This is handy for landing as you can really get all the power out on final. I like to keep it at 900+ for normal operations. 1300 when leaned sounds too high!

I'm following Jeff Schultz http://www.sonex604.com/aerocarb_thoughts.html and using a 2.5 needle.

It's worth remembering this fuel system does NOT like rapid throttle opening. It will always lean out if the throttle is rapidly opened, at a given mixture setting. So on short final, PERHAPS a leaned idle is a good thing. Go around, both knobs in, as slowly as your adrenal gland will allow. Detonation on full throttle is what we really want to avoid.

I did a lot of research on AeroInjector needles. The three I received with the unit where numbered 2, 21/2, and 3.
The higher the number the more the taper angel increases. It's just a few minutes of angle but makes a big change. What I found was that the sweet spot on the needle for wot was at a thickness of approx. .080, no mater what the needle number was. And the sweet spot on the needle for idle is very near the beginning of the taper. So the problem is getting the idle sweet spot in the right position after wot is set. I think your problem is the same as mine was with the lower number needles, the lower angle sets the two sweet spots further apart. I ended up with a #3 needle which when set for wot allowed the engine to idle without a mixture change. I took off with the mixture set about halfway out. I leaned a little more at cruise for economy. Exhaust gas temp. 11-1200. I landed, taxied, and never touched the mixture again until the next flight.

The EGTs matter. They aren't meant to be absolute values however. You can still use the relative info for tuning, which is what they are meant for.

If I'm reading your post correctly, full rich on TO gives you 6.6gph, but a 1/4 turn out on a vernier mixture causes the engine to stumble. If that's the case, I believe you are too lean. I can TO with half mixture, albeit with reduced power (38") with the Aerovee Turbo. I only know that because I did it by accident. As soon as I went full rich before rotation the MAP went to 41".

I also believe full rich should give you more than 6.6gph. I don't have a fuel flow sensor, but I believe it should be over 7.0gph if not 8gph. Someone else here might chime in. At the end of the day you can always lean, but you can't go higher than full rich.

A lower needle number will give you what you look for. I went to 2.5 to give a better idle mixture. As mentioned above, I still lean almost all the way out for ground ops, I might have 1/4" left to cutoff.

I had to buy a jeweler's file and "customize" the needle before I got any satisfaction.

I truly believe you are either blessed with an Aeroinjector with a small orifice, or damned with an oversized hole. In any event, the ramp on the needle is the key.

Happily edge finishing parts here for Chett 2.0 @Area 51%

Wow, lots of responses. Some helpful some not so, but all appreciated.

The 1/4 turn I referenced was on the needle adjusting leaner. No matter.

As promissed I went out and checked the idle (with intensionality and focus). It was not as I had stated. It was more like 870 Lean - 700 to dead on rich. So I guess I'm a bit thick headed sometimes but I did go fly then upon landing, while at full rich, I set and locked the throttle at 1000 rpm and shutdown. Then I reset the idle screw to where the slide was. After, I went flying again and things were much better. Idle is richer than I want but it's steady and not dying. Only problem was upon landing the idle really seemed to be closer to 1100. Boy did this cause a float issue. I'll work on getting it closer to 900. If I can get the mixture better I'd like to get the idle to 800.

I get confused when I try to visualize the needle taper, slide & orfice. Usually, in my head, I end up thinking of the narrow end of the needle being for idle, which it's not. Any hoo, after a good night sleep, I'll re-read some of the responses.