Aerocarb Alternatives

[Onex107]

I might add, I do not make seasonal adjustments. And temps vary here from 95 to below freezing. The Onex is a rocket ship in cold air. Having the wot and idle set to run with the mixture out a small amount, it takes very little movement to compensate for air temp change.

[sonex1374]

Mervin,

I'm glad the podcast was helpful! I'll try to address your comments, below.

Could you take a moment to explain why you made this suggestion? I suspect the steeper angle on the needle provides more fuel at full throttle, and less at idle???

Correct. The greater the needle number, the "steeper" the slope of the taper cut into the needle is, and the more fuel flows at full throttle. This steeper taper results in a greater difference in the flow area between any two points on the needle, such as full throttle and idle.

A little more explanation of the needle is required to really understand....

**Warning....I'm going down a technical rabbit hole, so viewer discretion is advised!**

Each needle is cut from a cylindrical rod approx. 70mm long and 3.14mm in diameter. The needle fits into an orifice in the carb body, approx. 3.21mm in diameter (so even with zero taper some fuel can sneak by the needle and into the engine). The first 20mm of the needle is dedicated to attaching it to the needle carrier. The taper starts a little ways down from the end, and we'll call the start of the taper "station 0". The remainder of the needle tapers in a constant cut towards the tip, ending at "station 57" (i.e. 57mm down the needle). From my own tests, the engine will likely want to idle at a point close to station 0 (usually around station 3) - this lets a small amount of fuel pass from the orifice and into the intake. If you were to measure the needle at station 3 (or wherever you have your needle set at idle) you would find that the thickness of the needle is approx. 3.1mm. This is pretty similar on many engines.

Moving down the needle towards the wide open throttle (WOT) station on the carb, that spot will be approx. at station 35 (i.e. station 0 plus 35mm due to the total travel of the carb on a 35mm AeroCarb). At station 35, you can really see the difference in needle thickness due to the steeper taper. For example, a #1 needle is 2.2mm thick, while a #3 needle is only 1.9mm thick. This is a difference of over 30% in flow area!

Each engine will need a certain amount of flow area at WOT, and thus will need to be adjusted to a certain thickness to achieve that flow area. My Jabiru needs a needle thickness of 1.75mm (e.g. 9 gal/hr), while Gary's AeroVee might need a thickness of 2.17mm (e.g. 6.5 gal/hr). On my #3 needle that 1.75mm thickness occurs at station 40, and that makes idle occur at station 5 (40mm - 35mm for my carb travel), for a thickness at idle of 2.97mm. You can clearly see that at idle, my 2.97mm has more flow area than the recommended 3.1mm, and thus my engine idles rich at that setting. This is exactly what happens....and to many others as well. I compensate by leaning on the ground, and it works great.

What would happen if I switched to a #1 needle? To get 1.75mm at WOT I'd adjust that needle to station 57 (the very tip, so clearly not practical!), and that would place the idle position at station 22 (57mm - 35mm) with a resulting thickness of 2.52mm, yielding way too much flow, and a horribly rich idle mixture! If I adjusted my #1 needle to idle nicely, WOT would be at station 35 with a thickness of 2.19 - this is only 60% of the flow area I need, and the engine would be severely lean!

You can see from all this that getting a needle profile is all about getting the right amount of fuel flow area at WOT and idle, and that means getting the right taper between those two points. In practice, it's slightly more complicated than this, but this approximation works well enough for understanding's sake.

So how do we summarize all this? Well, adjust your mixture so that WOT is running well, ensuring enough flow to meet the engine's needs (as shown by your WOT EGT's), and let the idle flow area/mixture fall where it may. If this happens to be excessively rich, then you need to switch to a larger number needle to lean out your idle. With the larger needle you'll still adjust the needle to the same thickness setting (and thus flow area), but that will just happen to be at a different station number, and thus a different idle station. If you go too big on your needle number, the engine will get excessively lean at mid-range and lower throttle settings and may not even idle for being too lean.

I noticed a further problem in setting the needle height. When I go to set the locking set screw, it pushes down on the needle assembly, pushing it lean.

Having the needle carrier move on you as you tighten the locking screw messes everything up. Any approach that stops that is OK, such as a custom-designed needle carrier, deformed threads, holding the needle from moving in the carb throat, or other methods people may have developed. I think deforming the threads is the easiest, but in any case, you need to do something.

Jeff

[Friesen5]

Thanks Jeff and others who responded. Your explanations and recommendations are excellent, I think they should be added as an addendum to the Aercarb manual! I also think the manual should include information about peening the screw threads, though the peening might better be done when assembled at Sonex. I will install needle #3 asap.

Mervin Friesen
Sonex 122
Jab 2200
320 hours

[Sonex1517]

Thanks Jeff and others who responded. Your explanations and recommendations are excellent, I think they should be added as an addendum to the Aercarb manual! I also think the manual should include information about peening the screw threads, though the peening might better be done when assembled at Sonex. I will install needle #3 asap.

Mervin Friesen
Sonex 122
Jab 2200
320 hours

I kind of doubt that Sonex tech support would endorse my peening the threads. It's experimental so we all do things differently and that is a dual edged sword.

[kmacht]

Peening the threads is exactly what sonex recommended a few years ago when I asked about the needle moving when tightening the set screw. Unfortunately the fix only works for a little while as the peened steel threads wear away the aluminum housing threads and everything is loose again after a number of adjustments. I have heard of people putting a ball bearing between the setscrew and the needlescrew to fix the issue but havent tried it yet.

Keith
#554

[lpaaruule]

I have minimal experience with the AeroInjector, and haven't flown with it, but my 3300 is running well on the ground with it.

I had been using the #3 needle, but found that it was too rich at idle.

I'm using a #2 needle. I reshaped the needle as an option suggested by Sonex tech support. I marked where the full throttle point would be on the #2 needle, and filed from that portion to the tip, so the thickness in that range was about a 0.1 to 0.2mm thinner than the #3 needle. So there is a smooth "step" on the needle, so my engine runs rich enough at WOT.

It's running well in all RPM ranges. Once I'm flying I'll probably tweak it.

-------------------------------------

Edit: I ended up changing back to the #3 needle. I was getting inconsistent fuel flow at WOT, and thought that it might be due to the modified needle. It turned out that I had used more fuel than I thought during the runups, and I only had about a gallon of fuel left in the tank. But to my surprise, what affected WOT fuel flow even more was my fuel vent tube. I was seeing just over 10 gph when I first went to WOT, but a few seconds later it would drop to 9.4 gph. I replaced my vent line with an ID 50% larger, and now my fuel flow stays steady at WOT.

I'm going to keep running with the #3 needle because I think my modified one might have been too lean for cold weather starting.

[Friesen5]

Thanks for your reply, Robbie. Peening the threads was suggested to me by Kerry Fores, which I did yesterday.

My point in my previous suggestion is that I did about 4 adjustments before I discovered the moving needle issue. Had I known about this issue prior to tuning, I might have arrived to a tuned position earlier. I can see that the less patient builders would quickly be frustrated by not getting tuned following the manual because of this issue. That may also lead to bad mouthing a good product on the forums. Why not put it in the manual so that it is part of the tuning process? That said, product support from Kerry is excellent and quick.

Mervin Friesen
Sonex 122

[SonexFactoryTech]

Peening the threads of the needle carrier is part of the assembly process for every AeroInjector. It is not mentioned in the manual as 99% of the time the factory procedure eliminates any chance the needle carrier can migrate once the set screw is installed. Each AeroInjector is tested for this on assembly. It is not mentioned in the manual as we prefer to not have builders hammering on their threads unnecessarily and without guidance. That can cause severe deformation which can destroy the threads in the slide. We have been peening the threads on each AeroInjector for about 5 years now, but time goes so quickly it may be 7 years.

What is often forgotten is there are older products out there that are just entering service, sometimes with the original owner, often not, sometimes with the help of the manual, all-too-often not. There are builder-modified units that have changed hands. Sometimes the new owner knows the unit has been modified but most of the time they do not. There is no one-size-fits all solution to any problem.

Blue Skies....

Kerry Fores

Sonex/AeroConversions Tech Support
Sonex Aircraft LLC

[builderflyer]

I, for one, have forever been frustrated in the inability to select a needle size and adjustment that will satisfy the recommendations of both Aeroconversions and Jabiru Au. One issue from the beginning is that Jabiru Au. says that when generating more than 70% power (say WOT), do not exceed an EGT of 1256 F. On cylinder nos. 5 and 6, I typically see 1420 F or higher at WOT and no needle size or adjustment will bring this down to Jabs specs. Some Jab3300 drivers apparently experience this same phenomenon while others don't ???

The only solution I've seen that works is the one presented by John Monnett and that was to not measure the EGTS on the rear cylinders as I guess he had seen the same occurrence on the factory prototype. So for several years I did just that, my pangs of anxiety were reduced to a minimum and the engine didn't seem to mind at all.

Has anyone else seen this happen and find a remedy for it besides reducing the throttle setting which has never worked for me unless I reduce the throttle so far as to not be able to climb very well?

Art,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,,,,Jab3300 #261 (old Aerocarb, modified and used only by me),,,,,,,,,,,,,,,,,,,,12 years and 540 hours later

[NWade]

I, for one, have forever been frustrated in the inability to select a needle size and adjustment that will satisfy the recommendations of both Aeroconversions and Jabiru Au. One issue from the beginning is that Jabiru Au. says that when generating more than 70% power (say WOT), do not exceed an EGT of 1256 F. On cylinder nos. 5 and 6, I typically see 1420 F or higher at WOT and no needle size or adjustment will bring this down to Jabs specs. Some Jab3300 drivers apparently experience this same phenomenon while others don't ???

Art -

I know Jabiru makes the engine, but I would view *any* EGT temperature limit with a healthy dose of skepticism. First off, EGT values can vary a lot based on the exact placement of your sensor in the exhaust pipe. Secondly, Exhuast Gas Temperatures vary quite a bit from moment to moment, as the engine pulses pass by any given point in the exhaust system - so your probe is only giving you a "rough average" of what its seeing. Third, all temperature readouts are susceptible to slight differences in calibration and dissimilar-metals junctions.

I highly recommend reading Mike Busch's EGT Myths Debunkedarticle. Mike is a highly-respected A&P who does EAA webinars every month and speaks at Oshkosh every year.

EGT readings are a good _relative_ measure of when you're running richer or leaner, but in and of themselves they don't tell you a lot about what's going on inside the engine. CHT measurements are usually a far better way to understand how much stress the engine is under and whether you are running the possibility of damaging parts.

Take care,

--Noel