Replies to This Discussion

Permalink Reply by John Austin on October 12, 2019 at 3:55pm

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

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Permalink Reply by Gilbert Cardwell on October 12, 2019 at 7:24pm

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?

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Permalink Reply by John Austin on October 12, 2019 at 8:07pm

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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Permalink Reply by Don Burkholder on December 21, 2019 at 9:06am

Hi Gilbert. You are losing just over 15% of your horse power at that altitude. 3300 engines should achieve 2800 – 3000 RPM at the aircraft’s best climb speed. Full Power Static RPM Recommendations All 3300 Variants RPM 2600 – 2800.

Do you have electric carb heat?

Some K&N's incorporate a small velocity stack inside which helps. You will have to measure to find how much room you have and search for about 200 cfm capacity air filter.

Airflow would be best if supplied vertically downward then smooth radius elbow to carb entry when using remote filter. This was much easier to do on my shorter 2200 as my radiator hose elbow had a 5" straight shot to carb.. Hope this helps.

Merry Ho Ho

Don

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Permalink Reply by Greg Sloan on August 20, 2020 at 2:57pm

sorry i know this is a little older post    did the Hose Help or what did you end up with? i am in a similar  situation  Jab 3300 in Zenith 750 max RPM is 2800 and home field at 5140' 

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Permalink Reply by Gilbert Cardwell on August 20, 2020 at 5:01pm

Hi Greg,

Scat hose did not help much.  I wound up ordering a fiber glass molding from Arion Aircraft (Jabiru engine - North American Dealer). I no longer have the part number, but its a piece made for the Jabiru aircraft that provides a 45 degree bend to connect the Bing carb to the air box in the Jabiur air-frame.  It fits the space I have in my 601 and it includes internal ducking vanes to smooth the air flow around the bend.  Had to do a slight modification of those vanes to make the molding fit over the mouth of the Bing carb.  That modification amounted to grounding out a small gap in the vanes with a dremel tool.  Have put about 30 hours flying time my airplane since installing the fiber glass molding.  I get acceptable, but not spectacular climb performance from my base at 5200 ft., even with 2 people on board and in the 8000+ ft density altitude conditions I've been flying in recently.  Arion Aircraft has recommend that I slightly flatten the pitch in my ground adjustable prop, but I haven taken the step yet.

Hope this help.  Let us know how it goes.

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Permalink Reply by Greg Sloan on August 22, 2020 at 12:10am

Thx very much

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