Thoughts on the AeroCarb and AeroInjector, Revisited

  Occasionally discussion about the AeroCarb draws some heated opinions. I've written about slide carbs before, and just recently I posted a few of my thoughts on the Website. I've copied my comments from that discussion below.

I've tuned several AeroCarbs and AeroInjectors, both my own and others, and have formulated a few conclusions over time. First off, small things make a big difference. Because of this, it' s very difficult to compare people's experiences directly because their setups often contain numerous small differences that may be having major impacts on their ability to tune.

The list of "small things" that comes to mind includes:

- Fuel system specifics (line routing, gascolator or filter, proximity of the fuel line to the exhaust pipes, location of the flow sensor). I think the best thing you can do is to simplify the routing as much as possible and keep flow rate as high as possible (i.e. reduce constrictions). Pay attention to the FWF instructions Sonex publishes (no high spots in line, no place for vapor to form or collect, consider ditching the gascolator). The fuel flow sensor location seems to cause trouble as well. I mount mine right after the tank shutoff valve and it reads perfectly and stays cool there, as opposed to mounting it close to the carb where it vibrates, heats up, and may collect vapor).

- Cowling setup (size of inlets, "smoothness" of intake openings, size of the exhaust openings, extra openings like NACA vents, internal blast tubes, size of the oil cooler inlet) - the cowling specifics affect the airflow inside the cowl, which has a big impact on the carb itself. High or low pressure into the carb throat affects airflow into the engine, and thus mixture. A "hot bubble" that sits over the carb or fuel line can heat soak the line, create burps or change mixtures at random times. Too much pressure in the lower part of the cowl (i.e. extra inlets, oversized oil cooler inlet, or too small exhaust outlets) stifles cooling flow and messes up the carb (this is frustrating because it will tune fine on the ground but change tuning once in flight).

- Air filter (size and type, clean vs dirty, even high vs low density altitude). Sonex recommends tuning without the filter. If you get it tuned right without a filter, then add the filter and things change, you probably need to change something to improve airflow thru the filter.

- Needle selection (old brass needles that came with the early AeroCarbs vs the new SS needles, diameter of the needles, diameter of the orifice in the slide itself, "smoothness" of the taper cut into the needle). I've measured several old needles and some of them mic smaller than the newer SS needles. This will make tuning hard because a sloppy fit of the needle in the orifice will always allow fuel to flow around the perimeter of the needle itself, making the engine run rich at idle. The quality of the taper also matters - a rough "step" where the taper begins, milling ridges in the machined surface, or other "imperfections" will change how fuel flows through the carb. Don't believe me? Take two needles of the same value and slightly modify one (sand or polish the surface, round over a corner or two, something seemingly small and innocent) and you'll see that that needle will tune differently. The new AeroInjectors use a nylon (?) bushing insert at the orifice to provide a tighter more consistent fit around the needle. The carb is difficult to tune if the fit between the orifice and needle is "sloppy", say from the wear of a few hundred hours run time and vibration.

- Technique. Everyone has their preferred technique to tell whether their carb is running rich or lean, and it can vary quite a bit. You can't go with just one or two indicators (like plug color, max RPM, black exhaust or smoke, EGT, EGT rise on leaning, or general feel of the engine), you need to consider multiple indicators and merge them all into a comprehensive assessment of where you are in the tuning range. Consider the case of being *very* rich. You go to WOT and see your EGT's going over redline. You conclude that you are overly lean because of the high temps, and you richen even further. Had you confirmed with something else (leaned the mixture knob while at WOT, observed smoothness and max RPM while doing so), you might reach a different conclusion, like the excessively rich mixture is still burning in the exhaust manifold and that's why the EGT's are high.

Best thing to do while tuning is to play with the mixture knob at various throttle settings (idle, 1-2 mid-range settings, WOT) and see how the engine reacts. If you get overly lean the engine will cut out abruptly. If you can lean the knob by 50% and see the engine running better or no real visible change, you're likely still rich. If the engine cuts out before you reach the 50% point, you might be close to the sweet-spot and have to proceed carefully.

The reason for this requires some visualization. The mixture lever is like a rotating ball valve. As the valve is rotated fuel flow is reduced, but not in a linear fashion. The first portion (0-25%) of the mixture throw has little effect on flow, and is somewhat linear in response. Then as the mixture is further rotated (25%-60%), it starts to choke the flow down in a somewhat linear in response. The last portion of the throw rapidly cuts off flow to almost nothing. It's almost impossible to find the exact right amount of leaning when you are in the last portion of the mixture throw because it tends to go from OK-flow to not-enough with just the smallest change.

So here's my abbreviated tuning method. Start with the recommended needle (#2.5 AeroVee, #3 Jabiru 3300). Inspect it carefully for manufacturing defects and how it fits into the orifice in the carb body. Make sure when you re-install it in the carb that you set it like the manual states (flat towards the engine, set screw in the proper location and tight). Lock the needle carrier with the set screw, and note whether the carrier "creeps" when you tighten the set screw (you have to watch the needle carefully to tell if it's happening). Once all this is done, then install the carb onto the engine. Run the engine at idle or slightly higher rpm and lean with the mixture knob. It's almost certainly rich, but note how much you have to lean the knob to get it running decently, and when the engine cuts out (ideally around the 75% point of the mixture knob throw, at slightly higher than idle rpm). Repeat this for a mid-rpm point and note the smooth-running point and cut-out point. You may need to lean slightly (1/8 turn) just to get to this point if you can still lean past 75% or so without cutting out). Only then try to run a high rpm or WOT. Got to WOT and then note the max rpm. Then lean the mixture knob and note the response (rpm increase, smoothness increase, or cut out point). It won't take more than a few seconds to make these observations and determine if you are still rich at WOT. Adjust the needle by 1/8 turn (or less if you're getting close) and try again. When you get to the point that you only need to lean by 25% or so before the engine starts to bog down or cut out, you're getting close and might be ready to high-speed-taxi test / fly it to further assess. If at any point running at WOT that you can't lean the engine slightly to improve things and smooth things out, you might be too lean already and need to re-assess. The final fine-tuning will come after a few test flights.

The AeroCarb / AeroInjector is a simple device, but you can't expect it to be totally automatic. You trade the operational simplicity of a bing/zenith/marvel (where you don't really touch it in flight) for the dirt-simple mechanics of the aerocarb and the ability to lean easily in flight. The aerocarb works with gravity flow, resists ice, is easy to mount on the engine and feed air into (no air box, carb heat, cobra head), and is inexpensive. Is it perfect? No. But it does work well if you understand it and eliminate potential problems before hand.


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Updated: 27 Feb 15