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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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 Posted: Mon May 29, 2006 3:48 am Post subject: 912 idle speed adjustment |
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During our annual inspection we balanced the carbs on the 912UL and
adjusted the idle speed to 1400 rpm after the oil temperature was up to
120F. With the carbs. balanced the idle is much smoother than before.
However, after flying the airplane for about an hour with the oil temps
up into the normal operations range around 215F, when closing the
throttle the idle speed went down to well under 1400 rpm.
My question: Must the engine be up to normal operating oil temp
(190-230F) when adjusting the idle speed?
Thom in Buffalo
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Thom Riddle
Buffalo, NY (9G0)
Don't worry about old age... it doesn't last very long.
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joeing701(at)internet.is
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| Posted: Wed May 31, 2006 5:13 am Post subject: 912 idle speed adjustment |
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Hello Thom.
I have the same engine and just did the balance adjustments on the
carbs, but the gage showed me that the timing was off. Not sure how to
adjust that on this engine? But the idle speed can not be adjusted below
1800, because the engine will not run at lower speed. My adjustments
were done on a hot engine, i.e. normal oil temp.
The gage I used were the same as sold by Lockwood and CPS.
I would appreciate any ideas on how to adjust the engine so that it may
run at lower idle speeds such as 1400 rpm.
Thanks,
Johann G.
Thom Riddle wrote:
| Quote: |
<jtriddle(at)adelphia.net>
During our annual inspection we balanced the carbs on the 912UL and
adjusted the idle speed to 1400 rpm after the oil temperature was up
to 120F. With the carbs. balanced the idle is much smoother than
before. However, after flying the airplane for about an hour with the
oil temps up into the normal operations range around 215F, when
closing the throttle the idle speed went down to well under 1400 rpm.
My question: Must the engine be up to normal operating oil temp
(190-230F) when adjusting the idle speed?
Thom in Buffalo
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Wed May 31, 2006 5:43 am Post subject: Re: 912 idle speed adjustment |
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Johann,
Before our annual inspection our engine was idling (a bit roughly due to vaccumm inbalance) at 1400 rpm with no problem, other than roughness. My old Titan Tornado, sold recently, had the same engine and it idled smoothly at 1400 rpm. It was that way when I got it and I never touched the carbs.
During the inspection we put the dual vaccumm gauges on the engine and noticed that the vaccumm readings were different by about 2 inches so we balanced them quickly but the idle was not smooth until we repinstalled the balance tube. We then adjusted both idle speed screws the same amount to get it down to 1400 rpm. All this was done after warming up the engine on the ground to 120F oil temp.
After flying the airplane the next day, when fully warmed up, the idle speed was well below 1400 rpm. Our next step, this afternoon will be to increase the idle speed to perhaps 1600 rpm at 120F and see how it does after flying it to normal temps. Final adjustment will be made right after the flight while it is near normal operating temperature.
It is my understanding that the Rotax 912 series engines have no way for the owner/operator to adjust the timing. It is fixed at the factory and not adjustable.
Your inability to get the engine to idle smoothly at 1400 rpm is due to carb vaccumm inbalance. Use the gauges according to the Rotax manual to do this. Do NOT expect it to idle smoothly while doing this because it probably won't without the balance tube connected to both carbs. If you make sure the two vaccumm gauges read the same, when you put the balance tube back on it should idle smoothly at some rpm, then adjust each idle speed screw the exact same amount to maintian the balance and change the idle speed. The purpose of using the vaccumm gauges is to make sure that both carbs are opening the same amount at idle.
My original question was merely about what temperature the engine should be running at to adjust the idle speed. Apparently, to do it right, it must be up to normal op. temps., not just 120F.
Thom in Buffalo
FAA Powerplant Mechanic - but relatively new to Rotax w/ multiple carbs.
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peter.rees01(at)tiscali.c
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| Posted: Wed May 31, 2006 12:45 pm Post subject: 912 idle speed adjustment |
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Can I please ask - where did the figure of 1400 RPM come from?? I was under
the impression that the minimum recommended idle speed was 1700. The Rotax
service manual specifically states that idle speeds that are too low will
result in gearbox wear / damage. Are we setting ours too high or are you too
low?
Peter
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Wed May 31, 2006 2:13 pm Post subject: Re: 912 idle speed adjustment |
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Peter,
It is in the Rotax 912 series Operators Manual in Section 10.1.1) Operating Speeds and Limits (912 UL/A/F).
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Thom Riddle
Buffalo, NY (9G0)
Don't worry about old age... it doesn't last very long.
- Anonymous |
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ami(at)MCFADYEAN.FREESERV
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| Posted: Wed May 31, 2006 2:52 pm Post subject: 912 idle speed adjustment |
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On some aircraft (eg Europa) a fast idle speed will result in too much
residual thrust and difficulty in landing within a reasonably short
distance. A low idle speed helps in this temporary situation, although the
engine does not then have to be left at a low idle speed once the landing is
complete.
Duncan Mcf.
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