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  #1  
Old 09-04-2018, 07:48 AM
mwardle7 mwardle7 is offline
 
Join Date: Oct 2009
Location: Utah
Posts: 59
Default Rotax Performance at High Altitude with Different Fuels

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.
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  #2  
Old 09-04-2018, 09:37 AM
Piper J3's Avatar
Piper J3 Piper J3 is offline
 
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Location: Hinckley, Ohio
Posts: 1,017
Default

Just wondering... Have you weighted the Bing floats to see if they are in spec?
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Jim Stricker
Hinckley, Ohio
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2004
80 hrs Flying Aeronca Chief 11AC
1130 hrs Flying 46 Piper J-3 Cub
RV-12 E-LSA #120058 AWC Jul 2012 - Bought Flying Oct 2015 with 48TT - Hobbs now 392

LSRM-A Certificate 2016
Special Thanks to EJ Trucks
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  #3  
Old 09-04-2018, 12:32 PM
mwardle7 mwardle7 is offline
 
Join Date: Oct 2009
Location: Utah
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Quote:
Originally Posted by Piper J3 View Post
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. 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.
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  #4  
Old 09-04-2018, 02:58 PM
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Piper J3 Piper J3 is offline
 
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I have a scale similar to this… https://www.ebay.com/itm/Portable-20...4AAOSwI2xbjtf9 $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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Jim Stricker
Hinckley, Ohio
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2004
80 hrs Flying Aeronca Chief 11AC
1130 hrs Flying 46 Piper J-3 Cub
RV-12 E-LSA #120058 AWC Jul 2012 - Bought Flying Oct 2015 with 48TT - Hobbs now 392

LSRM-A Certificate 2016
Special Thanks to EJ Trucks
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  #5  
Old 09-04-2018, 06:46 PM
moosepileit moosepileit is offline
 
Join Date: Dec 2010
Location: Floyds Knobs, IN
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Default

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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Last edited by moosepileit : 09-04-2018 at 08:14 PM.
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  #6  
Old 09-05-2018, 05:18 AM
vic syracuse vic syracuse is offline
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Default 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
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  #7  
Old 09-05-2018, 07:59 AM
mwardle7 mwardle7 is offline
 
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Location: Utah
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Quote:
Originally Posted by vic syracuse View Post
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:


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


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.
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  #8  
Old 09-05-2018, 08:36 AM
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Piper J3 Piper J3 is offline
 
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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.
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Jim Stricker
Hinckley, Ohio
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2004
80 hrs Flying Aeronca Chief 11AC
1130 hrs Flying 46 Piper J-3 Cub
RV-12 E-LSA #120058 AWC Jul 2012 - Bought Flying Oct 2015 with 48TT - Hobbs now 392

LSRM-A Certificate 2016
Special Thanks to EJ Trucks
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  #9  
Old 09-05-2018, 09:03 PM
newamiga newamiga is offline
 
Join Date: Jan 2011
Location: Barneveld, NY
Posts: 180
Default

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


Quote:
Originally Posted by mwardle7 View Post
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:


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


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.
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  #10  
Old 09-06-2018, 03:22 AM
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Piper J3 Piper J3 is offline
 
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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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Jim Stricker
Hinckley, Ohio
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2004
80 hrs Flying Aeronca Chief 11AC
1130 hrs Flying 46 Piper J-3 Cub
RV-12 E-LSA #120058 AWC Jul 2012 - Bought Flying Oct 2015 with 48TT - Hobbs now 392

LSRM-A Certificate 2016
Special Thanks to EJ Trucks

Last edited by Piper J3 : 09-06-2018 at 03:25 AM.
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