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Rotax Performance at High Altitude with Different Fuels

mwardle7

Well Known Member
I live in Utah and fly through the Rocky Mountains on most cross country flights. I typically cruise at 10,500 or 11,500 (when I eventually get there), and in the summer the density altitude at cruise is around 13,000.

The performance of this little engine is impressive at altitude, but it does have its quirks.

I have noticed a peculiarity when flying at high altitude with automotive fuel, particularly ethanol fuels. When I fly with automotive fuel, I always have a fuel smell in the cockpit, which I have determined is coming from fuel venting out of the carb bowls. When I fly with 100LL, I do not have this problem.

On another note, I tend to run very rich at altitude. Regardless of the fuel type, my EGTs at cruise are around 1,100 degrees. Everyone I've talked to says to not mess with the stock jets, 1.5 turn on the idle jet, and needle clip positions. Given that my home field is at over 4,000 feet, I've questioned this advice but I've stuck to it. I have also accepted the advice to avoid the temptation of installing the HACman mixture control.

I've had occasion to run the Rotax near sea level twice. The performance and smoothness of the engine is awesome at lower altitudes.

If anyone has any insight into running the Rotax at high altitude, I would appreciate your wisdom.
 
Just wondering... Have you weighted the Bing floats to see if they are in spec?

That is a great question, Jim. I am on my third set of floats this year. :mad: My last replacement was a set of 861?188 floats in May. I?ll try and check and see if the new ?won?t sink? floats are sinking.
 
I have a scale similar to this… https://www.ebay.com/itm/Portable-2...369076&hash=item58b6dcc0ad:g:lX4AAOSwI2xbjtf9 $6.82 including shipping. Scale weighs to 0.01 grams. Rotax spec says a set of two floats needs to weigh less than 7 grams.
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14sdzd3.png
 
Anecdotal...

Had a 912 80hp Kitfox bought in summer from a 5000'msl field.

Moved to sea level. Was fine running with Stratomaster E1 happy numbers on 89 octane no ethanol mogas, ran fine until winter.

Would cough on touch and go in freezing temps, no it was not carb ice. Same with facet on or off and engine pump was also steady. Too lean going WOT.

Circlips were 1 notch from top/leanest.
Lowered 1 notch. Happy year round up to 7500 feet.

Maybe you are on 1 notch from bottom and need to drop the needle by raising the circlip one notch.

Assumes lots of common failure points are nominal and clean like floats and seats and vents.
 
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Move the clips

I think you got bad advice on not moving the circlips. They are adjustable for a reason. I have over 1400 hours behind the 912's in Kitfoxes I built, and I usually changed the circlips between Summer and winter flying in Atlanta every year. I even got to where I could do it without needing to resync the carbs. It really made a difference on performance, whether I was running auto fuel or avgas.
They only needed to be moved one step.

Your EGT's are running quite rich.

Try it. I think you will notice a big difference.

Vic
 
I think you got bad advice on not moving the circlips.

.......

Try it. I think you will notice a big difference.

Vic

Thank you, Vic. Given the caution from the tech support lines to from the various Rotax suppliers, I have been reluctant to mess with the clips, although it makes a lot of sense to do so. I'll give it a try.

I ordered this carb manual from www.bingcarburetor.com:
2dkbb5j.jpg


There's a main jet correction chart in the manual that looks like this:
2drckzl.jpg


Given that moving the jet needle clips only affects the throttle in the mid range, I'm wondering if it will make much difference at high altitude where I fly with the throttle wide open. Any comments on adjusting the main jet? The stock 912 has a 155 main jet. Using the chart, if I conservatively use an altitude of 3,000 feet and 68 degrees F, the multiplication factor is 0.97, which means I should be using a 150 main jet (in theory). Has anybody had any luck with these kind of adjustments? Obviously, such changes have to be done thoughtfully and conservatively.
 
I would say the thought process is correct? needle doesn?t effect high-end mixture because taper of needle is fully lifted from the main jet at WOT. Your EGT is quite low. I typically see ~1330F at 5400RPM @ 3000MSL. Makes sense to lean the main jet, starting in small increments, if always operating at high altitudes (including base airport). I?d do spark plug readings at each jet change ? maybe all four top plugs. Don?t forget thermal paste on spark plug threads per Rotax manual.
 
I flew my 912 powered Gobosh out of Colorado Springs for about 5 years. I was actually really impressed with the performance at altitude as well as the climb performance on high DA days. I regularly saw 10K foot DA in the summer. My mechanic did try adjusting the jet needle clip. I really didn't see an improvement from this change. I too had several sets of bad floats. I have recently moved to NY and agree that the difference in performance between being in the mountain west and the east coast are just incredible.

I will say that the best my plane has flown was after I had a bad float issue and some other carb issues at OSH two years ago and ended up having to leave the plane there and have the guys from LEAF come out and rebuild and bring the carbs back to bone factory standard. It runs great now.

Carl


Thank you, Vic. Given the caution from the tech support lines to from the various Rotax suppliers, I have been reluctant to mess with the clips, although it makes a lot of sense to do so. I'll give it a try.

I ordered this carb manual from www.bingcarburetor.com:
2dkbb5j.jpg


There's a main jet correction chart in the manual that looks like this:
2drckzl.jpg


Given that moving the jet needle clips only affects the throttle in the mid range, I'm wondering if it will make much difference at high altitude where I fly with the throttle wide open. Any comments on adjusting the main jet? The stock 912 has a 155 main jet. Using the chart, if I conservatively use an altitude of 3,000 feet and 68 degrees F, the multiplication factor is 0.97, which means I should be using a 150 main jet (in theory). Has anybody had any luck with these kind of adjustments? Obviously, such changes have to be done thoughtfully and conservatively.
 
I miss-spoke in my post #8 above regarding EGT's. See pic below taken in cruise flight at 7500MSL with EGT's at 1200F.

Fuel flow and oil temp are a little high because airplane just came out of extended climb...
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s4xoy8.png
 
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Given that moving the jet needle clips only affects the throttle in the mid range, I'm wondering if it will make much difference at high altitude where I fly with the throttle wide open.

I think this is a misconception about the 912 carbs.

There is no connection between the throttle and the carb piston. The carb piston is moved by the difference between the venturi pressure and the pressure at the carb inlet.

This is influenced by the density and volume of air flowing through the carb.

So the carb is on the main jet at low altitude WOT. As the density decreases the airflow is less able to lift the piston even at WOT so the carb will transition to the needle jet. That is how the carbs get their altitude-compensating function.

I don't know at what actual power setting the transition occurs but it's not a throttle position. It must be related to (approximately) mass airflow. I don't know enough about the carb function to be sure but I think the piston probably moves proportionally to air density but the square of the airflow. The needle taper must therefore be a compromise, so the altitude compensation isn't perfect.

It would be interesting to rig a camera looking down the air filter to see the piston move with different altitudes and throttle settings.

In regards to the original question, it sounds like the auto fuel is starting to boil due to the reduced pressure at altitude.
 
There was a 3 part series by Brian & Carol Carpenter about the Rotax carburetor
in the Jan, Feb & March 2017 issues of Sport Aviation (Experimenter section).
PDF copies can be downloaded from EAA HERE.
You will have to sign in with your username and password.
Search for Keyword, "carburetor".
Set the search dates between January - March 2017.
 
Off topic, but part of the fuel system: the latest EAA Sport Aviation has a good article on how the mechanical fuel pump works.
 
In regards to the original question, it sounds like the auto fuel is starting to boil due to the reduced pressure at altitude.

This is one of the things I've suspected, but didn't know it could actually happen. It makes sense given the circumstances. I've noticed that the fuel smell is more pronounced when the ambient temperatures are hotter, and particularly when I use fuel with ethanol. Given my understanding that ethanol fuels can boil at a lower temperature than non-ethonol fuels, and given that 100 LL boils at a higher temp than automotive fuels, this just make sense to me.

So, if fuel boiling is the issue (which I'm not sure it is - this is all theoretical at this point) then what is there to do about it? Not using auto fuels, flying lower, and cooling the fuel somehow seem logical. Cooling the fuel seems the most difficult, impractical, and the highest risk due to unforeseen risks/downsides. How could it be tested? I think the simple answer is to just use 100 LL when I'm going to fly high and hot. I may also just avoid ethanol fuel; I'm fortunate to have access to 91 octane Ethonol fuel about 10 miles from Salt Lake International where I am hangared. But it is nice to understand, diagnose, and confirm before choosing a course of action.

Thanks to those who have posted helpful info about carb tuning. Brian and Carol Carpenter's articles are awesome. If you ever visit Rainbow Aviation, Brian 3D printed a giant Bing 64 Carb with all of the moving parts. It's a masterpiece. Here are a few other Bing carb articles that I've found informative:

Using Choke in Carburator Tuning
Tuning the Bing Carburator
Understanding the Mid-Range Circuit

I'm going to experiment with 150 main jets and weigh the floats this weekend. We'll see how that goes.
 
Boiling in the carb bowl has been a problem in the past. My 914 has carb drip trays whose main purpose is to keep fuel off the exhaust in case of a carb bowl leak. A secondary purpose is to act as a heat shield, to block heat from the exhaust pipe from the carb bowls. This is particularly a problem when you shut down after a flight and go to restart soon after. The heat will cause the fuel in the carb bowls to evaporate and starting is difficult.

Have a look at the parts manual to see what the drip trays look like and do a search on the Rotax site for examples others have fit.

Brian's talk on Bing carbs at Oshkosh was really good. I'd talk to him about your problems.

Jim Butcher
Rotax 914 in europa
 
Here are a few other Bing carb articles that I've found informative:
Using Choke in Carburator Tuning
Tuning the Bing Carburator
Understanding the Mid-Range Circuit

Beware: Those articles appear to be about the Rotax 2-stroke carbs, which operate on a different principle to the 912 carbs. I would not rely on tuning information from those articles to apply to the 4 stroke carbs.

So, if fuel boiling is the issue (which I'm not sure it is - this is all theoretical at this point) then what is there to do about it? Not using auto fuels, flying lower, and cooling the fuel somehow seem logical.

If auto fuel is causing a problem, I think the simple answer is to use a fuel that is formulated for use at altitude i.e. avgas. From what I have read, as the fuel approaches vapor pressure limits bubbles will form in the float bowls, which means the fuel is less dense and the floats sink, giving the same symptoms as bad floats.

Brian and Carol Carpenter's articles are awesome. If you ever visit Rainbow Aviation, Brian 3D printed a giant Bing 64 Carb with all of the moving parts. It's a masterpiece.

Are these the articles?
https://electricmotorglider.com/2017/02/01/bing-64-cv-carburetor-part-1/
https://electricmotorglider.com/2017/03/01/bing-64-cv-carburetor-starting-carb-part-2/
https://electricmotorglider.com/2017/04/11/bing-64-cv-carburetor-idle-circuit-part-3/

I agree they're good. They also confirm that you are probably not on the main jet at altitude:

As the aircraft gains altitude, the air density diminishes whereas the fuel density remains the same. This inevitably results in a rich mixture. The CV Carburetor on the other hand, senses the reduced atmospheric pressure on the lower half of the diaphragm, and the piston lowers in the body of the carburetor. This happens automatically even though the throttle valve position is still in the full open position.
 
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