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budyerly(at)msn.com
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 Posted: Tue Oct 12, 2010 10:54 am Post subject: Rotax 914 manifold pressure increase questions for the exper |
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OK guys. I need help on something Dean at Lockwood and I can't explain.
On climb with the manifold pressure set at 34 inches, the Airmaster holding
5500 RPM and starting altitude of 1000 MSL.
Observation:
During the climb the manifold pressure rises with the throttle locked firm.
By 5000 MSL I have to pull the throttle back about 1/4 inch.
By 10,000 MSL I have to to pull the throttle back another 1/4 inch.
As altitude is increased, the MP slowly creeps up at a rate of 1 inch of
manifold pressure per 5000 feet. Conditions at takeoff 17C, standard lapse
rate or near to it. Fuel used 100LL first test and Mogas RON 93... Stock
914, no intercooler good balance, Airmaster AP332 and AP420.
I don't do mush higher altitude flying any longer, but I never recalled this
characteristic in the newer 914s. Of course I have an old 914 in which
every AD ever written applies to, so confidence does not run real high.
Although the engine performs at sea level perfectly, it sounds great, and
has now in the last 50 hours been incredibly reliable and fuel efficient.
Temps are all fine. I can climb all day with the oil at 190F and the
Cylinder head at 220 at max continuous..
Please give me feedback if you have experienced similar indications on your
914 climb outs and or if your MPs are stable. Reading the maintenance and
operations manuals, the throttle once set at a specific RPM (that the prop
controls), the MP normally stays quite steady (granted, at these outside
temps the engine does not encounter thermal reset so I do not expect that
the computer is lowering my MP/power at low altitude and as temperature
drops in the climb, the throttle position calls for a higher MP be delivered
and the gate opens and MP increases). Only problem with my past experience
is the throttle will creep on some aircraft due to throttle friction
control. Perhaps I have just never paid attention closely before.
Proposed Troubleshoot were working on.
Dean at Lockwood and I have discussed it and we feel the following is the
best course.
1. Check the MP gauge. (I will put a Tee in and run two simultaneously and
pinch one off and the other to check.) Let's face it, if the MP gauge
leaked I would have decreasing MP readings.
2. Ambient Pressure and Airbox Sensors bench OK, at low static. Remove and
take pressure reading vs resistance/voltage readings to check operation
using a vacuum pump.
3. Check the sensor tubing of course.
4. Check leaning valve for operation.
5. Fly to Lockwood and climb out to altitude, record data, then download
computer via dongle program. (Yes my old engine needs a special adapter to
download the computer memory.)
Although I have always been able to troubleshoot my own engines, this one
may be just fine and normal, but I still find it a bit unusual. Of course
my real rub is I hate to have to pull the power back and note the rate of
climb drop, which is the reason for my quest.
Thanks guys,
Bud Yerly
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rparigoris
Joined: 24 Nov 2009
Posts: 805
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| Posted: Tue Oct 12, 2010 11:38 am Post subject: Re: Rotax 914 manifold pressure increase questions for the e |
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Hi Bud
"(granted, at these outside
temps the engine does not encounter thermal reset so I do not expect that
the computer is lowering my MP/power at low altitude and as temperature
drops in the climb, the throttle position calls for a higher MP be delivered
and the gate opens and MP increases). "
Just to be certain, the wastegate closes to increase MP.
Did you check that the wastegate and cable is very free and the cable is adjusted where it has the spring pushing the wastegate closed properly?
Does the wastegate servo seem to be functioning properly? No hee bee gee bees more than normal?
Perhaps if you have a spare servo you could try one if nothing else seems to work. I seem to remember on a old style high time 914 someone ended up finding a bad servo. I can't remember if he disassembled the servo and cleaned up the wipers or replaced it.
Also check your plumbing, perhaps you have a wonky leak that somewhat changes with temp and pressures. Inlet and outlet to carbs first. Perhaps your MP gauge is leaking and somewhat sealing at altitude? Can you fly with dongle and see if you are really gaining MP when climbing?
Good luck
Ron Parigoris
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rparigoris
Joined: 24 Nov 2009
Posts: 805
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| Posted: Tue Oct 12, 2010 2:22 pm Post subject: Re: Rotax 914 manifold pressure increase questions for the e |
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Hi Bud
Noodeling your problem a bit:
Did you check to see if the airfilter is clogged/restricting?
Ron Parigoris
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h&jeuropa
Joined: 07 Nov 2006
Posts: 650
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| Posted: Wed Oct 13, 2010 4:26 am Post subject: Re: Rotax 914 manifold pressure increase questions for the e |
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Hi Bud,
Can you use the monitoring program while in flight or record some data for evaluation? MP is the same as air box pressure less the drop across the carb, so if air box pressure isn't rising, it's likely a guage problem.
I've also found the guys at Rotax in Vancouver very knowledgable and helpful with 914 questions.
Jim Butcher
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budyerly(at)msn.com
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| Posted: Wed Oct 13, 2010 7:10 am Post subject: Rotax 914 manifold pressure increase questions for the exper |
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Jim,
Yes, it can be used in flight. True about the MP vs. Airbox, that's why I
think I may need to fly down to Lockwood.
I was hoping I had a leak in the carbs or MP gauge something akin to that
but it doesn't seem to have a leakage anywhere, and besides that would drive
the MP down not up with altitude...
What I am seeing Jim is a steady rise in MP with altitude increase with a
fixed throttle. That is weird as the manuals don't seem to indicate that
should happen except for the high airbox temp cutback.
Regards to Heather,
Bud
---
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