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Tuning the AeroCarb Ė What Iíve learned over the last 400 hrs

I have been flying Sonex airplanes since 2009 and as of 2016 have approximately 400 hours in them. All my time has been behind engines running the AeroCarb and AeroInjector, and I feel like Iím in a good position to comment on the tuning and operation of those devices. What follows is a discussion of my tuning method, lessons learned and thoughts overall on the keys to success for other builders.

What is an AeroCarb (or AeroInjector)?

The AeroCarb is a replacement carburetor manufactured and sold by the folks at Sonex Aircraft under the AeroConversions product line. It is designed to be simple, lightweight and adjustable in flight for mixture changes. If you review the factory literature on it and listen to the history of the AeroCarb, youíll learn that this particular device has design roots in several much older carburetors (Posa in the 1970ís, even older ones before that). The concept of slide-valve carbs are well understood, allowing Sonex to refine the product over the years to the current iteration of the AeroInjector. The AeroInjector is simply a refined AeroCarb, so Iíll use those terms (somewhat inaccurately) interchangeably.

The Aerocarb has 3 main advantages:

1) Simple design and low parts count (i.e. low cost to manufacture). At $400 itís far less expensive than many other carbs in use for aviation.

2) Simple in operation. It requires low head pressure (i.e. gravity flow works in most cases), eliminating fuel pumps. It requires no sensors to the engine or atmosphere, thus eliminating those as potential points of failure. Itís not prone to carb icing like many other carbs. No one will go so far as to say itís ice-proof, but the design alone overcomes to most significant reason carbs ice-up: the butterfly and venture.

3) In-flight mixture adjustment. Allows for tuning over a variety of density altitudes, at the cost of slightly higher pilot workload.

 

Before you tuneÖ.

Before delving into the tuning process and trouble-shooting tips, letís review the pre-requisites of your setup.

1) Control cable setup. Check the routing of the throttle and mixture cables. Ensure they have no bends sharper than a 4Ē radius. The throttle cable is more critical in this respect, and doing this wrong can contribute to the slide sticking when the engine is running. Carb sticking is caused by the engine manifold pressure ďsuckingĒ the slide against the AeroCarb housing. Any grit, debris, or scratches in the slide or Teflon liner will only get worse with the engine running. Cable routing thatís too tight can cause added friction inside the cable, requiring more force to move. When thereís no manifold pressure ďsuctionĒ you might be able to overcome the friction easily, and may not realize itís even there. Likewise, ensure the cable doesnít come close to an exhaust pipe. If the cable overheats it will melt the inner liner of the housing and cause friction. Route cables with at least 4Ē of space from exhaust pipes, otherwise wrap the section of cable in insulation and/or install a radiant heat shield to block the heat.

2) Pull-open configuration. Early versions of the AeroCarb were set up for push-open (i.e. push the slide open to full throttle). Later Sonex changed to push or pull, now the latest AeroInjectors are pull-open only. Pull-open has several advantages (like reducing throttle sticking), and thereís a reason why Sonex switched over. If you want to use a Vernier throttle knob youíll need a direction-reversing bellcrank. My advice is use pull open and a throttle quadrant lever, or make really sure that the bellcrank mechanism is sturdy and slop-free. Youíll also need to save room on the firewall to mount the bellcrank, so plan ahead.

3) Mixture arm opens fully. The instruction state to ensure the mixture lever arm swings thru 45 degrees when fully open. Make really sure you get this right. If itís off by a little bit youíll never get full rich on your carb.

4) Fuel system inspection. Youíll have a frustrating time tuning if you have an underlying problem in your fuel system. Ensure that you have sufficient fuel flow at the carb before you start. I suggest getting 150% of full throttle fuel flow with 2 gal in the tank (10 gal per hr for AeroVee, 15 gph for Jabiru 3300). This requires some effort. Minimize restrictions and fittings, and use large-diameter fittings and hoses (AN6 or AN8).

5) Air Filter. Clean, and large enough to breathe easily. Consider using a K&N filter. They have less restriction to flow and are reusable.

6) Needle selection and orientation. Confirm you have the right needle installed - 2.5 for and AeroVee, 3 for Jabiru 3300. Double check the orientation of the needle flat. Iíve seen more than one installed incorrectly. The flat goes towards engine.

7) Needle set to factory starting position. Read the manual and adjust accordingly (start of taper just at edge of carb sidewall).

8) Set screw in place. The needle carrier can migrate due to vibration without the set screw in place. Always re-install after making any adjustments.

9) Intake leak. Double check your intake couplers are tight and there are no leaks. Youíll chase settings even with a small, intermittent leak. Double check this!

 

My Tuning Process. Engine Start.

Once I verify all the above items, then I start the tuning process. The engine will start and run over a wide variety of mixture settings while at low throttle, so following the instructions will almost certainly get the engine started. The biggest danger here is over-priming the engine and flooding it. The method I use to start is to shut the fuel tank ball valve, position the throttle to idle plus a ďsmidgeĒ (e.g. 1/8 open), pre-position all my switches and get the key ready, push the mixture knob full rich, and then turn on the fuel valve. I give a 3 second delay to prime, then crank immediately. If you delay too long fuel will soak into the filter and clog it, creating an overly-rich setting that wonít start. Worst case scenario is the now fuel-soaked filter catches a stray spark from the exhaust and catches fire (itís happened!). Best case is that you wear your battery down while cranking and have to try again after recharging. Better to under-prime a little and crank a little longer Ė it will catch after a second or two if the prime is a little light.

Once the engine is started, let it warm up for a minute or two. The engine will very likely be running rich, but this is OK for now. After things warm a bit, you can proceed. (I use CHT of 150 deg F as a good indication things are ready.

Leaning Test.

I like to call this next part the ďleaning testĒ, as Iíll use the mixture knob to help me determine how well the mixture is set. Determine might be inaccurateÖ infer or deduce might be a better description. The test is outlined below.

1) Observe the start-up and see if thereís a black smoke puff on engine start. This can be hard if youíre by yourself, but it gives you an indication of how rich your starting mixture is. This is not an essential part, but can help fill in the picture. The smoke will be along a continuum: large black puff Ė very rich. Grayish-back puff Ė getting leaner. Very small light-gray wisp Ė pretty good mixture. No smoke Ė idle set really well, but full throttle is probably way too lean.

2) EGT rise response. While at high-idle (1200-1600 rpm), observe the EGTís to ensure they are within limits. At idle they should be at the low end of the range, say 900 deg F or so. If youíre above 1000 or 1100 deg you are likely going to be lean already at full throttle. Pull the mixture knob out slowly to about 1/4 of its travel. Observe the EGTís Ė they should rise. Lean to half travel Ė EGTís should continue rising, and the rpm should increase slightly and the engine sound smoother. Continue leaning to 3/4 travel. EGTís should rise slightly, but should be getting close to peaking (although you wonít know what the peak is initially, so this can be hard to tell). Lastly, continue to lean the last 1/4 of the travel while watching EGT and rpm. You want to identify the peak rpm and EGT before the engine quits from going too lean. Donít worry, you canít hurt the engine by over leaning at idle.

The goal with the EGT rise test is to figure out how much leaning you should be doing to the mixture (at idle) to get to peak EGT and rpm. If itís around half to 3/4 lean, youíre in the ball park. If the engine dies at 1/4 lean youíre WAAAY too lean. If it dies at nearly full out youíre too rich. This is why you want a high-idle Ė because at low idle you wonít see much change of EGT, and the mixture can be leaned brutally lean, then it dies. Thatís normal, and not really any help in tuning you engine.

3) Make a Mixture Adjustment, or Not.

Based on the high-idle test, you either adjust, or proceed to the next step. If youíre in the range I described above (half to 3/4 lean before dying), youíre ready to go to the mid-power leaning test. If youíre not in the range, youíll need to make an adjustment of the needle.

4) Adjusting the needle.

Adjustments should always be made in small increments. I recommend 1/4 turn initially, reducing to 1/8 turn as you think youíre getting close to the sweet-spot. Always double check the direction you are turning: itís easy to get confused and go the wrong way (then you canít figure out why itís not working!). Leaner is ďneedle screwed INĒ, and the wrench is turned clockwise. Always put the set screw back in before running then engine to ensure the carrier doesnít move on its own.

One thing to keep in mind. At idle, you can probably turn the needle a full turn, or two even, and the symptoms wonít change that much, so donít spend a lot of time at idle trying to tune the carb! Get it in the range of mixtures, then move on to the next throttle range leaning test.

5) Mid-rpm Leaning Test.

Repeat the leaning test at a mid-range rpm. I like to use 2000-2400 rpm. The process goes exactly like before, but the EGTís will be higher, and the leaning will be more responsive. Youíll also notice that as you approach the peak rpm and just before the engine dies that EGTís might be uncomfortably high. If youíve over 1300 deg F you may want to act fast to limit the time spent running this lean, passing quickly through the last little bit of mixture range until the engine quits. You still want to see how far out the mixture needs to be pulled to get to the sweet-spot. Around half-out may be getting close. Less than that and you might be too lean at full throttle. If you see a large increase in EGTís (200-300 deg F) from full rich to peak lean, youíre too rich. If you pull the mixture more than 3/4 before it peaks and dies, youíre too rich. Adjust the needle as needed.

6) Full-throttle Leaning Test.

As you get the mid-rpm mixture zeroíd in youíre really just ensuring the mixture is rich enough at full throttle to control your temps. Remember, your mixture will likely end up being rich at lower throttle settings, and thatís OK. You really really donít want it too lean at full throttle though.

The key to the full-throttle test is NOT to set it to full throttle rpm and then start leaning the mixture to see how it responds. Thatís hard on the engine, and hard on the pilot! You want to set it A LITTLE RICH so you have some margin for error on your first takeoff. I define ďa little richĒ as getting a distinct EGT and rpm rise at full throttle when the mixture is pulled out, even slightly (Sonex says 100 deg F). If itís set right, youíll see this at about 1/4 mixture out, and the engine will have died (or EGTís gone way too high) before you get to mixture half out.

7) High-Speed Taxi Run.

This test might be considered optional, but I think itís a useful test. The goal of the high speed taxi run is to test the engine as close to actual takeoff conditions as possible, without actually leaving the ground. It will happen fast, and if youíre not careful youíll accidently take off, or lose control on the runway, so use your best judgement.

There are two main goals for the high-speed taxi run: 1) Test throttle response and engine spin-up to see if the engine lugs down or stumbles. 2) Ensure EGTís and rpm are where you expect them, based on what you saw while doing the Full-throttle test. If theyíre off, you might need to go back and re-run the full-throttle test another time, as you might be too lean. When you add in the acceleration of the plane down the runway and the prop biting into the air it seems to show a lean situation better than standing still. Likewise, if the engine misses and stumbles, but smooths out as power and rpm come up, you might be too rich. Smooth running is more telling than EGTís. If your EGTís are in the 1100-1300 range, and the engine is smooth, youíre probably ready for flight.

8) Flight test.

Once the engine is running well at full throttle, youíve gotten a high-speed run or two to verify acceleration, throttle response, smoothness, and EGTís, youíre ready to test fly the engine. Things should go fine during the first part of the takeoff because youíve already practiced that with the high-speed taxi test. However, the mixture will most likely not be set perfectly, and youíll likely see an indication that itís not by either the EGTís running high on takeoff (above 1300 could mean youíre a bit lean still) or some lugging and stumbling on climb-out.

Thereís a good way to tell how close to optimal your mixture is set. If you reduce power by 20% or so, say from 3200 rpm to 2800 rpm, youíre likely to move into a range that the engine is naturally running more rich (remember the part about the engine running rich at idle). If the engine improves with a slight throttle reduction, you might need to richen the mixture setting. Conversely, if you reducing power doesnít help smooth things out and you EGTís are still under control (1100-1300), try leaning the mixture know slightly while still at that slightly reduced throttle setting. If leaning improves the engine, youíre still running rich, and maybe are too rich for full power operation.

9) Conclusion.

The best way to really fine-tune your mixture is to play with it in flight at the slightly-less-than-full-throttle setting. If youíre set to an optimal needle setting, you shouldnít need much if any leaning at this setting to make the engine purr. If you canít tolerate any leaning, then youíre too lean. If you lean considerably to smooth out, then youíre rich. Flight test, then adjust the needle, and repeat. Go slow when adjusting the needle. I recommend no more than 1/8 turn when youíre getting close to the sweet spot. And remember, slightly rich is better than lean, but you can always lean the mixture a bit by either reducing power slightly (richen) or pulling mixture knob (leaning), so use those tools to help determine exactly how your carb is set, and what you need to change.

 

Common Mistakes.

There are recurring things that Iíve observed that cause all sorts of frustrations. Iím outlining those here to help prevent these things from complicating the tuning process.

1) Tuning entirely with the cowling off, then trying to fly without verifying the setting with the cowl in place. The cowling will have an effect on the engine tuning, but can be a pain to install and remove when doing frequent adjustments. However, at a minimum you should do a high-speed taxi run or full power run-up to verify the setting are still good with the cowl in place.

2) Restricted air filter. A dirty filter can really choke down airflow, and that will richen your mixture. Ensure that the filter is clean before you tune, or re-tune. I recommend using a K&N filter. The K&N filters breath better for the same size (dimensions) filter as a paper filter. They can be cleaned and reused easily, and best of all they work really well on the larger Jabiru engines, where the recommended paper filter is really too small.

3) Too large of adjustments to needle. Itís easy to blow right through the sweet spot with a 1/4 turn, so make small adjustments as you get close. I think some builders test out the engine at idle and (correctly) deduce that the mixture is very rich. They then proceed to make large adjustments to lean the engine, then get it set hopelessly lean for full power operations. This is another reason to not spend a lot of time at idle trying to tune.

4) Mixture arm not AT LEAST 45 deg at full rich. If you eyeball this and itís a little short you could be restricting fuel flow at full throttle, when it needs it the most. It likely will never show itself as a problems at lower throttle settings.

5) Needle fits ďsloppyĒ in fuel orifice. This isnít so much a builder mistake, but a wear and tear issue, or some inconsistent needle manufacturing. The old style brass needles were often undersized, and varied greatly between needles with the same number on them. Differences in diameter, or a sloppy fit of the needle in the orifice, will affect the mixture, especially at idle. New style needles are stainless steel, and are much more consistent. If you have an old carb with brass needles, do yourself a favor and replace them with new ones from Sonex.

6) Wrong needle used. If the needle is not matched to the fuel flow requirements of the engine, youíre never going to get good results. Use a #2.5 for the AeroVee, and a #3 for the Jabiru 3300. Using a needle that is too small will require you to adjust the needle really rich for full power operation, making idle run WAAAY to rich.

7) Basic engine setup wrong. Sometimes tuning canít be done due to an unrelated engine setup problem: intake leaks, ignition timing, and cooling problems come to mind. Double check your intake couplers. Double check your ignition timing, or just run on the primary ignition for your early test flights to ensure an advanced secondary timing isnít causing problems (like high CHTís). Cooling problems can cause the CHTís to run too high, with can tempt you to richen the mixture to cool then engine. This only works to a point. Better to address the cooling problems directly and leave the mixture alone.

8) Sticky throttle. You wonít experience this until youíre airborne, and maybe on final needing power for a go around. Better to take all the preventive steps early on so you never have to solve it later after having a scary experience with sticking.

9) Non-standard setup. Stick to the way the manual tells you to setup the carb. Donít try to add ram air for increased power. If you use a remote mounted filter connected with SCAT tubing, be careful that the turbulence caused by the SCAT tubing isnít causing problems. How can you tell if it is? Run the engine in a standard configuration first, get it working well, then connect up the scat tube and remote filter or carb heat or whatever and see if it suddenly starts acting up. Determined to run a fuel pump and/or fuel pressure regulator? Ensure you have steady flow, of sufficient flow rate, not susceptible to vapor lock. Youíll need to ensure the fuel system is bullet-proof, and it may require some critical thought and observation.

 

Tips for trouble shooting:

When youíve done all the items above and you still have problems, itís time for some trouble shooting. Below is my short-list of tips for troubleshooting.

1) Re-read the manual. Sounds silly, but often times the answer is in thereÖyou just missed the significance of what you read.

2) Verify all your assumptions, especially the ones you donít even know youíre making. This one is really hard to do. We make all kinds of assumptions, often subconsciously, that might get us into trouble. Assumptions like: Of course the fuel system is free of restrictions; This fuel line wonít get that hot and cause vapor bubbles; I cut my cowl exits per the plans so they must be large enough; My carb is already too rich so thereís no way I need to richen it more; and so on. Challenge them all!

3) Start from factory settings, then move slowly. No major deviations. No large needle changes from the factory starting positions (change slowly so you donít overshoot). No creative re-designs on a new airplane Ė start with the factory setup then change after the basic reliability is established.

4) Donít spend too much time adjusting on the ground. Flight test is far more useful!

5) Stay close to the airport while you build confidence. Be smart about your flight testing, and stay within gliding distance of the runway. Youíll breathe easier, and be more productive. After a few hours of staying close youíll have the engine purring well, and will be ready to move on with the rest of the flight test program.

 

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Updated: 29 Sep 16