Jabiru 3300, Bing carburetor and air intake system on in the Zodiac XL

Having flown my Zodiac XL about 600 hours with my Jabiru 3300 and Bing carburetor I must say it has had it good points and bad.

The short story is that initially the performance I got was quite poor. With some minor changes, performance improved to the point where it seemed acceptable. However problems remain which are perhaps serious.

Now the longer story...

First, in defense of Jabiru and the Bing I will say that the firewall forward kit I got with the engine did not properly address installation in the Zodiac. Essentially the kit seemed to encourage installation techniques that produced bad results and were in direct conflict with the installation recommendations in the Jabiru engine installation manual. Note too that the manual itself has improved over time. The most recently released manual (dated August 2009) is the best yet.

To make matters worse, when I built my airplane I was new to the entire process, so I hired a certified A/P to install my engine. He did a mostly reasonable job but actually did not read the installation manual. I assumed he knew what he was doing. I have learned: hay, it’s your life, read the manual no matter who is on the job.

The firewall forward kit I got failed to address the following two issues mentioned in the installation manual:

1) Do not connect scat tube directly to the carburetor.
2) Avoid a 90 degree bend from the side in the intake tube coming into the carburetor.

In addition, I have learned though independent study and personal experience that scat tube in general offers significant turbulence and drag in the intake system, especially when bent. If you choose to use it, use as little as possible and avoid sharp turns.

I no longer have any scat tube in my cold air intake system. I almost never use my carburetor heat system so I am not worried about scat tube in that system. In your climate, perhaps you need carburetor heat, in my climate I don’t.

With my initial setup, which had scat tube everywhere, I found that the engine ran very rich at higher throttle settings. At full throttle the engine burned about 13 GPH – not exactly what I was expecting! Also at 10,000 feet the engine ran so rich that it shook the airplane badly at full throttle. I knew something was wrong so I started looking into it.

I learned that air does not like to turn corners. Because the air traveling along the outside of the bend has further to go than the air traveling along the inside of the bend, turbulence results. Scat tube is not smooth, especially when bent. The uneven walls of the scat tube create drag and turbulence. Inducing turbulence right at the carburetor intake creates problems, especially for the Bing carburetor which as sense ports at its opening.

I have read that A 90 degree bend in an intake system produce drag that is roughly equivalent to 3 feet of straight tube. Because of the way my intake system was put together, when I computed the total effective length of the tube in my cold air intake system it came out to about 12 feet! What a mess. No wonder the engine was performing badly.

My cold air intake system now uses only radiator hose. The hose is as short as possible with as few bends as I could manage without a complete re-work of the system.



Simply eliminating all scat tube and replacing it with radiator hose eliminated the high fuel consumption and added a very noticeable amount of power – it was not subtle. However a new problem showed up. At wide open throttle the EGTs were very uneven. To help reduce this problem I added a piece of aluminum inside the radiator hose running from the air box to the carburetor. The aluminum is centered in the hose, is vertical and is bent such that it runs down the center of the hose as it makes the 90 bend. This helps the air make the corner, reduces turbulence and has evened out the EGTs a bit.


All these modifications were made about 550 hours ago. EGT temperatures are still not as even as I would like and vary considerably with throttle and altitude. Different cylinders are hotter than others depending on throttle setting. On some days when I fly at about 1,800 feet MSL and have a certain throttle setting at least half of the EGTs get too hot. It is necessary to change altitude or throttle setting. About 150 hours ago I re-pitched my prop to make it steeper. That lowered my engine RPM by about 100 RPM. That has helped keep this annoyance to a minimum but it is still there.

I have noticed that on my engine it is the back cylinders which seem to have high EGTs. I have always suspected that this is because their intake runners connect to the intake manifold on the extreme left and right. The Bing’s main jet is in the middle and the Jabiru’s intake manifold is quite small so mixture is unevenly distributed to the cylinders. The two rear cylinders run lean as a result. Just a theory mind you…

About 80 hours ago I found a burned exhaust valve on my number 5 cylinder. It was easy to replace but got me to thinking. Perhaps that cylinder has been running too lean.

More recently oil consumption has been going way up. The engine now burns about a quart every 4 hours. When I replaced the burned valve I notice some scoring in the cylinder wall. I am not sure but at this point I am suspecting broken rings on #5 which might explain where the oil is going. In the next day or so I am going to run another compression check with the hope of learning what is wrong and where the oil might be going.

So, is it possible that in 600 hours with un-even EGTs with the tendency for the rear cylinders to run lean I have shortened the life of my engine? I don’t know – I am just wondering…

I have been considering one or more modifications.

1) Redesigning the air intake to eliminate the 90 degree bend from the side. This would be done with a custom air box intake system to replace the radiator hose.
2) Replacing the Bing carburetor with a Rotec TBI-40.

In summery one of the Bing’s strengths is also one of its weaknesses: the lack of a mixture control. I like the simplicity. I don’t like the lack of control.

I am attracted to the Rotec TBI-40 because of the claim that EGTs are more even. I am not looking forward to re-running my throttle cable, turning my coke cable into a primer cable and adding a mixture control.

On the other hand I can’t say I am thrilled at the prospect of an early engine overhaul either.

I wish I had more information…

Steve

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Comment by Robert Emery on March 15, 2013 at 1:51am
Hi Steve
At 110 hrs I am still having high no 6 cht and EGT in cruise Jab 3300 with Bing in my Zodiac 601 XLB.
I have tried all the usual, lip on lower cowel, rad hose intake, split ali in hose, i also have mixture control fitted.
I also tried lip on front upper cowel to catch air from rising prop.
My cht round 160 C max for no 6.
I have ordered the Cobra Head but dont know if it will fit.
May have to redesign air box.
I fly in Perth West Australia in high ambient temps,
Frustrating isnt it.
Bob
Comment by Stephen R. Smith on March 3, 2012 at 10:08am

Hello Thomas,

I purchased the hose from NAPA auto parts.  At the time the part number was 7905.

Steve

Comment by Thomas Spickermann on March 2, 2012 at 7:35pm

Hi Steve,

 

it's been a while, but I was wondering if you remember where you bought the radiator hose? The Scat hose that's supposed to go between the airbox and the Bing on my 750/3300 is 2 1/4" I.D., but I am having trouble finding a 2 1/4 inch rad hose with a 90 degree bend. Or did you use 2 inch and streched it a little?

Thanks, Thomas

Comment by Felipe Uribe Posada on February 3, 2010 at 1:45pm
Steve,

This comment is from a guy in Colombia, Where there is no one to refer to for experience with Jabiru 3300. I am right in the process of mounting the engine and the first thing I noticed was the carb to airbox link. Just from the gut i did not like scat tubing directly onto the carb, so I went shoping this morning for a radiator hose. what a coincidence that you mention it in your comments as a "do not" because air turbulence.

Thanks for confirming my modest engineering Knowlwdge.

Regards, Felipe Uribe zodiac 601 XL 6-7704
Comment by Stephen R. Smith on September 13, 2009 at 8:05pm
Craig,

Thanks for the links to Lance's pages.

My brother has purchased a cobra head and is experimenting with it. Today he learned It actually does not fit the 601 without modification which is disappointment #1.

I am leaning toward the Rotec TBI-40.

First I need to get my new oil cooler installed and re-assemble the engine - still waiting for parts.

Steve
Comment by Craig Payne on September 13, 2009 at 5:32pm
Link to Randy Stout's page on the Ellison on his 3300:

http://www.geocities.com/n282rs/ellison.html

-- Craig
Comment by Craig Payne on September 13, 2009 at 5:18pm
Lance Gingell is in your area and visits Cloverdale so you have probably already discussed this with him. He went through some of this with his XL/3300 in 2006 or so and installed the radiator hose. His log of his changes is here:

http://www.lancegingell.com/lance/Tuning_the_Zodiac.html

Lance tried the Jabiru fiberglass cobrahead and reported it made things worse.

I am unimpressed by the Aerocarb - it is basically a calibrated leak. It also (as stated by Sonex) works best (only?) in a gravity-fed fuel system. On a 3300 on a Sonex they have you remove the mechanical fuel pump. So if you install it on an XL you will need a fuel pressure regulator (something else to fail). I owned (but never installed) an Ellison throttle body injector (TBI). The Rotec folks acknowledge that their TBI is based on the Ellison. But the Ellison TBI is $800+. Both the Ellison and Rotec meter the fuel through a tube which spans the throat of the carb. The tube has a series of fine laser-drilled holes across the tube. The throttle plate progressively uncovers the holes while the mixture control varies the angle of the tube and holes in the airflow. Randy Stout replaced the Bing on his 3300 with the Ellison and was pleased with the results.

Craig
Comment by Stephen R. Smith on September 1, 2009 at 4:37pm
Hello Brad,

Our buddy Jake Reyna up in Idaho posted a blog a few days ago pointing out the new manual. That is how I learned of it. I download the manual and scanned it to see what was new. I placed a link to the manual in my original post (up above).

Steve
Comment by Brad DeMeo on September 1, 2009 at 2:53pm
Do you rework the air filter box? I haven't seen this before. Which installation manual did you copy to show this illustration? I am very interested in this.
Comment by Stephen R. Smith on September 1, 2009 at 9:41am
I don’t know anyone with either an Aerovee or the TBI-40 on a 3300. There out there; I just don’t have those contacts. From reading the websites of the two carburetors I am leaning towards the TBI-40. Apparently it’s a new product though, so I am reluctant to rush into it.

My brother Alan and I each have 601 XLs. We think we need to fix the airflow problems first because all 3 carburetors will benefit from this. If after we fix the airflow problem, if the Bing still does not get the job done, we will probably swap it out for the TBI-40.

We are going to use the cobra head offered by Jabiru on our redesigned air intake system. The hope is that by making the airflow more symmetrical, the uneven EGTs will be minimized.

Steve

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