Aeroinjector

[bipbuilder]

Have an aeroinjector on an aerovee non turbo. All indications are a lean mixture. Needle is the one sent in aeroinjector. Have turned it out 2 full turns. Egt is still climbing through 1300 degrees at 15 seconds of full throttle at full rich. I have checked mixture control arm travel and looked for leaks in intake. My main question is how far do you back out the needle before switching to the next richer needle.
P.s. last issue before first flight.

[sonex1374]

Dale,

Your AeroCarb probably has the #2 needle installed, so you can go to a #2.5 or even a #3 when using the turbo. If you have already turned the current needle out by two full turns then you're already at the point where you need the larger needle.

Keep in mind that EGT values alone at WOT are not a great way to determine if the engine is rich or lean. You'll need to look at the EGT and RPM changes as you lean with the cockpit mixture knob, look for black smoke out the exhaust, and measure the fuel flow (if you have that). All those put together will help identify if the engine is rich or lean. It's possible to be so rich that you have high EGTs (from fuel burning in the exhaust, I think), and you swear it's lean unless you saw the black smoke going out the exhaust or tried leaning it with the mixture knob.

In terms of absolute EGT values, here's what I've seen on my AeroVee (and Jabiru, actually). This is just my experience - so take it as a single data point. WOT EGT when tuned well is in the high 1200's to high 1300's, peak EGT is in the mid-to-high 1400s. At 75% power a good EGT is in the mid-to-high 1300s.

Jeff

[Onex107]

Jeff is exactly correct. There are two sweet spots on the needle. One near the beginning of the taper for idle and the other further out for the WOT setting. As you back out the needle to lean the WOT you are richening the IDLE spot by moving it away from the orifice at idle. As the needle number goes up ( 2 1/2 to 3 ) the angle increases which moves the two spots closer together. I find the WOT sweet spot to be at the .080 to .085 thickness of the needle. You can mark this point with a black marker and make the initial adjustment visually or, mark the beginning of the taper ( on the bottom with a file to give an accurate starting point ) and count the number of revolutions to get the setting. Knowing this number lets you get back to the same setting if you want to remove the needle. In my case I'm using the #3 needle set at 2 1/2 to 3 turns in from the "0" point. This set up has given me a good WOT and a good IDLE without making a mixture change at IDLE.
One important change is to eliminate the play in the ball socket joint in the needle holder. Mine was equal to a half a turn of the screw and made adjustment very difficult. Stick with it. It took me a while to come to these conclusions but they've been working fine for 315 hours of flying.

[Area 51%]

As you back out the needle to lean the WOT you are richening the IDLE spot by moving it away from the orifice at idle.

What am I missing here? Don't you richen (or lean) both ends of the spectrum with the same needle movement?

[NWade]

As you back out the needle to lean the WOT you are richening the IDLE spot by moving it away from the orifice at idle.


What am I missing here? Don't you richen (or lean) both ends of the spectrum with the same needle movement?

The needle has a fairly linear taper. But in reality the optimum fuel ratio at idle is not linearly correlated with the optimum fuel ratio at WOT.

You *are* adjusting both ends of the spectrum when you move the needle, but in order to get WOT correct, just about every engine winds up running excessively rich at idle (with the mixture knob pushed in all the way). You can pull the mixture knob out some at idle to lean the engine on the ground to compensate for this, and that’s what most people do.

—Noel
Sonex #1339
Aerovee Turbo

[sonex1374]

The simplest way to think about tuning the carb is to simply focus on getting the engine running well at WOT. Once this is setup and the engine is running well you can then make a determination if changing to a different needle is a good idea. Most installations running a #2 or #2.5 needle will find that the mixture is pretty rich at low throttle - this is easy to tell by leaning with the cockpit mixture knob. This test is best done at low rpm, but not necessarily at idle - something like 2000 rpm in cruise descent might be a good time to try this. If you can lean a whole lot at this low rpm and the engine runs better as a result, then the mixture is overly rich and changing to a larger needle will help this situation.

It's not intuitive why switching to a larger needle will make the low rpm leaner, but that's the way it works. Remember, you always tune the carb at WOT, so when you install the larger needle you'll adjust it to the exact same WOT fuel flow as the smaller needle (e.g the same "thickness" of the needle where it meets the fuel orifice).

The best way to truly understand this is to get out your needles and a micrometer. Find the point on the #1 needle where the tapered thickness is 1.7 mm and make a mark with a sharpie. Then measure 30mm towards the root end of the needle and measure the thickness at that point (should be around 2.6 mm). Next pick up a #3 needle and find the point on the needle where the thickness is 1.7 mm, mark it, then measure towards the root 30 mm and measure the thickness of the needle - it's going to be around 2.8 mm. This means that the #3 needle is thicker at this point, and flows less gas than the #1 needle! Once you hold them in your hands and see the effect of the taper this should make sense.

So there you have it. Get out the tools, make some measurements and record some figures, and this will make a lot more sense. Bottom line, always tune at WOT, and if the low rpm is overly rich, switch to a larger needle!

Jeff

[Rynoth]

As you back out the needle to lean the WOT you are richening the IDLE spot by moving it away from the orifice at idle.

What am I missing here? Don't you richen (or lean) both ends of the spectrum with the same needle movement?

I agree that the wording appears to be backwards. Backing out the needle will enrichen all aspects (WOT and IDLE), and screwing it in will lean all aspects. Switching needles will change the relationship between WOT and Idle, with a higher # needle giving a leaner mixture at idle for a given WOT mixture setting (and/or a richer WOT for a given idle mixture setting.)

[Onex107]

You guys are right on. I said it backwards. Wouldn't it be great if an adjustable needle could be made with a screw in the middle so both ends could be adjusted by the same needle?