Hi All.  I have a 601 with the Jabiru 3300 engine installed. I recently removed a Rotec TBI from the engine and replaced it with a stock bing type 94 carburetor ordered from the factory.  To make a long story short, I have traced a low static RPM problem (2430 max at WOT) to the rubber radiator hose (picture attached) used to connect the carburetor to the air box.  If the hose is connected to the carburetor BUT NOT connected to the air box, just hanging open in the air, I get the low 2430 max RPM value in a static test.  However, if I completely remove the hose I get the much more reasonable and expected 2750 RPM value in a static test.  

I have now replaced the rubber radiator hose with a scat tube connection (picture attached).  With the scat tube installed and the air filter installed and the cowling installed I get a marginally acceptable 2680 RPM in a static ground test. 

More inflammation: I have recently cleaned the air filter.  The air box is the standard part supplied with the Jabiur FWF kit.  The radiator hose is smooth on the inside but probably has too sharp a bend.  Bend is the same as shown in the picture when connected between the carb and the air box.

My question is:  does anyone know of an option other than a rubber hose or a scat tube that can be used in the very limited space between the carburetor and nose wheel strut on the 601 which provides better air flow to the bing carburetor?

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That rubber hose bend does look a little too sharp.  I'm running an AeroConversion's "AeroInjector" TBI carb now, but back when I ran the Bing, I, too, used a rubber smooth bore duct.  I simply went to an auto parts store and rummaged through their bins until I found a near-90 degree hose with a little smoother bend than you have pictured - worked great!

However, I also installed an airflow "straightener" between the hose and the carb.  This was an aluminum duct with vanes in an "X" cross-section to straighten the turbulent airflow exiting the rubber duct.  Pete, at Jab USA (now taken over by Arion Aircraft) gave me the airflow straightener - I don't know if Arion has them or not.

Your static tests all to be on the low side - do you have an adjustable prop that possibly needs a pitch adjustment?

John

N750A

Hi John.  Thanks for your comments.  I too rummaged through the bins at an auto parts store looking for something with a more gradual bend.  Didn't find anything. And I have another problem with the hose or scat tube approach.  Diameter of the air box outlet is larger than the diameter of the carb inlet.  So when the hose was clamped on the carb it had to be squeezed down some.  Think that squeezed section may have been at least part of the problem with the hose connection.  Had to make a short sleeve from another hose to "shim-up" the carb inlet so I could clamp the scat tube at both ends.  But, as you point out, I still have somewhat low static test RPMs (with everything installed and connected).

I do have a ground adjustable prop but I'm reluctant to change the prop until I'm more convinced I don't still have an airflow or carb jetting issue.  Big warning in my prop manual: "Don't mask a engine problem with a prop adjustment"!

What do you think of my static test result of 2750 RPMs when nothing is connected to the carburetor?  Does that seem low for a 3300 running in CO at 5600 ft altitude?  What static RPMs where you seeing with your Bing carb and what do you see now with your  "AeroInjector" TBI?  I'm guessing your AeroInjector is on a Sonix airplane?  And, in that case, you don't need a tube connection to an air box?  Air filter sits directly on the end of the TBI because no carb heat required?

No, my 3300 is on my STOL 750.  AeroConversions is a division of Sonex, which sells the "AeroVee" engine, but ironically, the Jab 3300 seems to be the "engine of choice" for most Sonex airplanes!  So, there's extensive experience with the AeroInjector (the former, similar model was the "AeroCarb") on Jab 3300 engines.

I'm totally at a loss to comment on what you should be seeing at your 5600' field altitude ... I simply have zero experience - I'm in East Tennessee with a 1000' field elevation and typically see about 2850 or so static and can attain redline/3300 with wide open throttle in straight and level flight at 2000-3000 ft.

You're correct that I no longer have an airbox - the K&N filter clamps directly on the AeroInjector.  No need for carb heat since supposedly TBI carbs are immune to carb icing and I'm drawing air from under the cowl, so it's pre-warmed a bit, anyway.

You can read about my set-up here.

John

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