SonexBuilders.net

Posted: **Tue Jun 13, 2023 8:25 am**

BRS wrote:
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...

900 RPM is about right. 700 is too low, especially for a new engine. My engine is broken in, and I probably could idle at 700, but why would I? The chances of the engine quitting are too high.

Posted: **Tue Jun 13, 2023 10:42 am**

NWade wrote: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?

—Noel

This is thread-drift bate, but I'll bite.
Yes, I'm sure it's mixture and not hot lines as the condition is constant whether I've just started the engine and it slightly warmed or if I just landed. My lines are all well insulated though I do have a gascolator. About the gascolator, I've thought of removing it several times just because it is so hard to get to. But, call me old school, I want a low fuel point to check for water. Even though I'm hangarared and use 100LL. Hold over from having rented cessnsas for many years. I'd rather like to install a low sump in the fuel tank since it has such a perfect location but don't want to introduce the fuel-in-cabin possibility.

OK, back to our regularly scheduled program...

Posted: **Tue Jun 13, 2023 10:45 am**

karmarepair wrote:It's worth remembering this fuel system does NOT like rapid throttle opening.

Yes, having flown fuel injected engines for the past 10 years, going back to a carb was a bit of adjustment in this regard. Don't even have an accelerator pump on this thing! Modern Stone Age.

Posted: **Tue Jun 13, 2023 10:57 am**

I found this cool picture on another thread:

I assume you remember it! It would seem the #3 needle gives a quicker change from rich to lean. Isn't this what you were looking for?

Also, fwiw, I can move my throttle pretty quickly and the engine is not unhappy.

Posted: **Tue Jun 13, 2023 10:58 am**

Onex107 wrote:...
The higher the number the more the taper angel increases.... 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.

This was most helpful, 1X107. When I think about the taper my brain want's to think of the thin end as idle. Which really get things confused. I'll have to give this some thought and take a close look at those needles while in the carb. This would be so easy if the cowl did not have to come off for each little adjustment.

On another note, does anyone have a chart of full rich fuel-flows for their great-running Aerovee (or revmaster)?

Posted: **Tue Jun 13, 2023 11:13 am**

Bryan Cotton wrote:I found this cool picture on another thread:
..pic removed..
I assume you remember it! It would seem the #3 needle gives a quicker change from rich to lean. Isn't this what you were looking for?...

Ha! Thanks Bryan for bringing that image back. I looked just now for the original, seems to be long gone. Funny, there have been so many twists & turns on this sonex project that I've now come full circle and barely recognise the starting point.

Posted: **Tue Jun 13, 2023 8:40 pm**

Well I'm encouraged. This afternoon I swapped out #2 needle for #3. I had thought it might be too lean but the ground run seemed to indicate it will be close to spot-on once I get things adjusted better. Here is what I found while running (tied down).
Static RPM WOT 3180 at a whopping 8gph. It was smooth running and took acceleration well.
Static idle at full-rich was about 900 rpm then later 800 after WOT runup.
Static idle mixture Leaned was 1000 rpm+. I had not made any idle speed adjustments from before.

So as long as I get a rise when leaning at idle I think I'm good.

Before installing the #3 needle I marked each needle at .080" and at the end of the taper at the idle side.

Posted: **Wed Jun 14, 2023 9:51 am**

Now you've got it. As you can see, the two "sweet spots" get closer together. I don't know what the slide movement distance is, but the object is to get them both within the movement length so they end up in the proper position when the slide is closed. I found that the lower number needles, when adjusted for wot, left the idle sweet spot outside the idle position and I had to lean the mixture almost to cutoff to not stall the engine by being too rich at idle.
I even calculated and graphed the aeria of the slide opening, as it opened, to see if it wasn't linear. And it is. That wasn't the problem,

Posted: **Wed Jun 14, 2023 1:59 pm**

In regards to the gascolator. I found that I could not insulate the unit adequately, so I eliminated it as per Sonex. If the fuel system is as per plans, it is downhill all the way from the tank to the AeroInjector. So, if any water is in the tank, it will be at the AeroInjector right away. In support of that, after 20 years of flying a Cessna I never saw/tested any water. And, I didn't religiously fill the tanks after flying. If you fly often enough condensation doesn't have time to happen.

Posted: **Thu Jun 15, 2023 6:56 am**

Good job. Looks like you are getting the 8gph I mentioned and a good idle.

Like the other poster I too deleted the gascolator, I was apprehensive to do so, but it has worked out great. No burps. I also insulated the 12" of fuel line I do have post firewall with fiberglass wrap and then gold foil wrap.

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