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I am currently experimenting with a Sonex/AeroConversions "Aero-Injector" throttle-body carburetor on my Jabiru 3300 vs the Bing 94. The Aero-Injector is extremely simple, very light, gravity-fed, immune to carburetor icing, and has a conventional mixture control. So far, the weight savings for the total installation is 5.4 lbs! For those Jab owners with the Bing, let me assure you that mine was working perfectly - as many know, I just like to experiment, so there were no problems with the Bing on a Jab 3300!
There is extensive experience with the Jab 3300/Aero-Injector (previous model called "Aerocarb") in Sonex aircraft. Since carb heat is not required, the Sonex builders typically just clamp a K&N filter directly onto the carb and this has been quite successful.
The Jabiru USA firewall-forward kit for Zenith aircraft (750, Cruzer, 601/650) has a rather elaborate fiberglass airbox riveted to the firewall that incorporates a carb heat valve and a K&N filter. Here's a pic from the installation manual:
I was hoping there might be someone who either (a) happened to have weighed their airbox prior to installation or (b) have an uninstalled firewall-forward kit and could weigh their uninstalled airbox.
Since my airbox is mounted, I can't weigh it and am curious what the potential weight reduction would be if it was removed and a direct, clamp-on air filter was substituted. If anyone could provide me with that information, I would appreciate it! Also, if you do have a weight, please note whether the weight includes the K&N filter inside the airbox or not. I don't need the weights of the associated ducts, etc.
John "Can't Leave Well Enough Alone" Austin ;>)
Does your installation include a fuel pump and pressure regulator?
No - AeroConversions says the Aero-Injector works best strictly gravity-fed, which of course is no problem since the STOL 750 is a high-wing with fuel tanks in the wings. The Jabiru 3300 has a mechanical fuel pump, but it is normally removed when an Aero-Injector is installed and replaced with a block-off plate.
You'd think gravity feed would provide a stable fuel pressure but there are times when gravity gets canceled out. It's no problem on float bowl carbs because there is a fuel supply within the carb and fuel is drawn from within the level of fuel. Aerocarbs are gravity fed to the mixture port and are subject to changes in fuel pressure, "gravity". So, changes in fuel tank to carb angle, acceleration on takeoff, or turbulence can change fuel mixture.
Most of the time these mixture changes go unnoticed unless the effects are prolonged like what happens during stalls.
I had 2 deadstick landings (Corvair/aerocarb/gravity) that I attribute to fuel pressure loss (nose over) at the mixture port.
Didn't have the nerve to try for a third.
Seems the predominant opinion and experience in the Sonex crowd (many of whom do mild aerobatics) is that with the Aerocarb/Aero-Injector, there likely will be minor mixture changes with +g maneuvers and momentary -g maneuvers are tolerated OK, but prolonged -g could definitely be a problem.
I personally can't see in my usual flight profile where I would be pulling prolonged negative g's, however, it would seem prudent to (at altitude above the airport!) try some stalls to see what happens!
Are the Sonex guys using gravity or pump/regulator systems?
It is my impression the majority are gravity systems since Sonex/AeroConversions does not recommend pumps/regulators for typical installations.
Today I did some deliberate nose-overs to generate -g's (enough to lift me off the seat!) and the engine purred right through them! I'm going to wait until I'm done with the final tuning and then take it to altitude and try some more and also stalls with power off and on.
Glad to hear you tested for that and that there was no problem. I wish I had taken notes as to fuel levels in my tanks, etc. After it happened the first time I completely purged the fuel system to remove possible water or air that may of been trapped and did a fuel flow test through the system. I even went as far as to insulate all the fuel lines and gascolator to eliminate possible vapor locking. Went back up to test for that specifically and it did it again. That's when I terminated it's employment.
Now it's got me scratching my head again.
FWIW I was at about 1/2 tanks. My gascolator is mounted on the lower edge of the firewall - not insulated, but protrudes about 1/2 way beneath the fuselage so I'm pretty sure it's exposed to quite a bit of cool turbulent air tripped by the extended lip on my lower cowl. The fuel line is extremely short - it runs directly from the gascolator through a short firesleeved line to the fuel flow red cube and then through a short firesleeved line to the carb - total length probably 12-15" and a straight shot upwards to the carb with no extreme bends or high points.
But, "it ain't over 'til it's over," so like I said, I'll repeat the testing including stalls once the final tuning is done.
Some Sonex builders have had issues with vapor bubbles and "burps" where the engine misses, usually corrected by insulating the fuel lines and/or gascolator. I understand the Sonex's cowl is rather tight around the 3300. I'm hopeful that cowl heat will be much less of an issue in my STOL 750 - the 3300 is a light engine and the engine mount holds it relatively far forward from the firewall for weight and balance. So, it's relatively spacious between the rear of the engine and the firewall, and I extended the intake runner between the carb and the plenum to position the carb just aft of the distributors for clearance - this also moves the carb and fuel lines that much further aft from the radiant heat of the engine and exhaust:
(This pic was taken prior to firesleeving and connecting fuel line from red cube to carb, safety wiring the carb to the plenum, and with the airbox duct attached. I'm likely going to delete the airbox and attach clamp a K&N directly on the intake bell of the carb. )
None of the components in that area ever look like they get unusually hot or "cooked," so I'm thinking my carb and fuel lines will be relatively cool and not subject to vapor issues.
Have you considered installing O2 sensors and an air/fuel mixture gauge? It's a fairly simple setup that makes it possible to very accurately adjust the fuel/air mixture.
No, I'm in the KISS (Keep It Simple, Stupid) mode. When trying to balance 6 cylinder's EGT's with a single carb, there are compromises, anyway, so I'm happy with having at least a 6-probe EGT/CHT and a mixture knob! Haha!
Of course, the ultimate set-up would be fuel injection, but that's definitely not a KISS project!
This a.m. we had spectacularly beautiful wx in the Sequatchie Valley and the winds were calm. I took N750A up a few thousand feet above KCSV and did a series of power-on and off stalls. I also descended in a sinusoidal or stairstep manner, pushing-over to induce neg G's. Obviously, I couldn't sustain a neg G more than a few seconds, but I would recover to slow down and then push right over again.
Throughout all these maneuvers, the engine never missed a beat. However, this was with nearly-full tanks. If I get an opportunity, I'll repeat the series with near-empty tanks. However, like a lot of pilots who fly primarily for pleasure, it's comforting that it would be extremely rare to be caught in a situation with "near-empty" tanks! ;>)