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Low RPM on Takeoff???

 
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tmitton



Joined: 04 Aug 2014
Posts: 4

PostPosted: Mon Aug 04, 2014 4:11 pm    Post subject: Low RPM on Takeoff??? Reply with quote

Recently my 912ULS engine started only going to 5000rpm on takeoff with full throttle.

Previously it always went directly to 5500rpm or so upon application of full power. Now it only goes above 5000rpm once it has been flying for a few minutes. Once flying it will still go to 5500+ even in a climb so it isn't like it is just the climbing load on take off that is holding it back.

No changes have been made to the engine so I don't know what would cause the RPM drop. It is enough that takes offs now need 1000+ feet where as before 500-600ft was enough. There is definitely a noticeable difference.

I have checked the carburetor linkages - full throw to the stops there. Drained the gas in case it was old and replaced it with fresh new gas - no change. Tried it with half a tank of 100LL to give it an octane boost - no change. Checked the spark plugs - still clean after only 25hrs of use. Nothing stuck on the rads. Oil topped up.

So - any idea as to what might cause the drop in rpm?


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davidjoyce(at)doctors.org
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PostPosted: Mon Aug 04, 2014 10:21 pm    Post subject: Low RPM on Takeoff??? Reply with quote

Don't know whether 912S works the same way as 914, but a
leak of fuel/air from the carb fuel bowl will do just that
on a 914. David Joyce
On Mon, 4 Aug 2014 17:11:20 -0700
"tmitton" <tmitton99(at)gmail.com> wrote:
Quote:

<tmitton99(at)gmail.com>

Recently my 912ULS engine started only going to 5000rpm
on takeoff with full throttle.

Previously it always went directly to 5500rpm or so upon
application of full power. Now it only goes above 5000rpm
once it has been flying for a few minutes. Once flying it
will still go to 5500+ even in a climb so it isn't like
it is just the climbing load on take off that is holding
it back.

No changes have been made to the engine so I don't know
what would cause the RPM drop. It is enough that takes
offs now need 1000+ feet where as before 500-600ft was
enough. There is definitely a noticeable difference.

I have checked the carburetor linkages - full throw to
the stops there. Drained the gas in case it was old and
replaced it with fresh new gas - no change. Tried it with
half a tank of 100LL to give it an octane boost - no
change. Checked the spark plugs - still clean after only
25hrs of use. Nothing stuck on the rads. Oil topped up.

So - any idea as to what might cause the drop in rpm?




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kerrimikehickman(at)aol.c
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PostPosted: Tue Aug 05, 2014 7:54 am    Post subject: Low RPM on Takeoff??? Reply with quote

I had that exact problem w/912ULS...
-Findings: very small hairline crack/approx. 2mm long in very small indentation (origin unknown) in top of one carb housing...
(discovered after sending carbs to LEAF for routine ultrasonic cleaning/rubber replacement).


--Result: replacement of carb top housing led to normal engine operation at all power settings.
Cheers
Mike

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BARRY CHECK 6



Joined: 15 Mar 2011
Posts: 738

PostPosted: Thu Aug 07, 2014 8:08 am    Post subject: Low RPM on Takeoff??? Reply with quote

Sounds like a STUCK VALVE or STUCK RINGS.
Or a Temperature issue with the Mags. Temperature required to expand something.
Either way it is a ROTAX...  What else would you expect!
Barry
I am looking for a Zenith 601 with a Continental O-200 or Lycoming O-235



On Mon, Aug 4, 2014 at 8:11 PM, tmitton <tmitton99(at)gmail.com (tmitton99(at)gmail.com)> wrote:
[quote]--> RotaxEngines-List message posted by: "tmitton" <tmitton99(at)gmail.com (tmitton99(at)gmail.com)>

Recently my 912ULS engine started only going to 5000rpm on takeoff with full throttle.

Previously it always went directly to 5500rpm or so upon application of full power. Now it only goes above 5000rpm once it has been flying for a few minutes. Once flying it will still go to 5500+ even in a climb so it isn't like it is just the climbing load on take off that is holding it back.

No changes have been made to the engine so I don't know what would cause the RPM drop. It is enough that takes offs now need 1000+ feet where as before 500-600ft was enough. There is definitely a noticeable difference.

I have checked the carburetor linkages - full throw to the stops there. Drained the gas in case it was old and replaced it with fresh new gas - no change. Tried it with half a tank of 100LL to give it an octane boost - no change. Checked the spark plugs - still clean after only 25hrs of use. Nothing stuck on the rads. Oil topped up.

So - any idea as to what might cause the drop in rpm?




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Roger Lee



Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

PostPosted: Thu Aug 07, 2014 11:39 am    Post subject: Re: Low RPM on Takeoff??? Reply with quote

5000 rpm on take off would equate to approximately 5600 rpm WOT flat and level. That should be about where you want to be for the average person.

If the compression test is normal (I expect it to be) and the carbs sync up well, the mag drops are fairly equal and temps are all still what they used to be then it could be your rpm gauge. If rpms were to really fall off that much then there has to be changes going on somewhere else. You don't loose 500 rpm with no other affects.
If you have an air leak that should show up by syncing the carbs (do this at 3500 rpm) or by spraying some starting fluid around the intake manifold and carb sockets and if the rpm jumps up you have a leak.
Check the prop blades for equal pitch. If you can find nothing then rte-pitch the prop and see where that takes you.
I have to ask.
Are you sure about your numbers?


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tmitton



Joined: 04 Aug 2014
Posts: 4

PostPosted: Thu Aug 07, 2014 7:10 pm    Post subject: Re: Low RPM on Takeoff??? Reply with quote

Positive about the numbers. Re-torqued all the prop bolts to spec (fixed pitch, wooden prop) and nothing changed. Still 5000rpm WOT on the ground, 5500+rpm WOT in the air. This sounds like good numbers but it WAS 5400+rpm WOT on the ground last week!

Might try the starting fluid spray around the carbs next and see what happens.

Thanks,

Todd


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rickofudall



Joined: 19 Sep 2009
Posts: 1392
Location: Udall, KS, USA

PostPosted: Fri Aug 08, 2014 6:16 am    Post subject: Low RPM on Takeoff??? Reply with quote

Todd, Any chance you have an intermittent ground on one ignition system?

Rick Girard
do not archive

On Thu, Aug 7, 2014 at 10:10 PM, tmitton <tmitton99(at)gmail.com (tmitton99(at)gmail.com)> wrote:
Quote:
--> RotaxEngines-List message posted by: "tmitton" <tmitton99(at)gmail.com (tmitton99(at)gmail.com)>

Positive about the numbers. Re-torqued all the prop bolts to spec (fixed pitch, wooden prop) and nothing changed. Still 5000rpm WOT on the ground, 5500+rpm WOT in the air. This sounds like good numbers but it WAS 5400+rpm WOT on the ground last week!

Might try the starting fluid spray around the carbs next and see what happens.

Thanks,

Todd




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b.carl@sympatico.ca



Joined: 20 May 2008
Posts: 77

PostPosted: Sat Aug 09, 2014 7:09 am    Post subject: Low RPM on Takeoff??? Reply with quote

Air density change (temperature, humidity) will cause a difference in static
RPM, but 500 is a lot. Also check your air filter.
Carl

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tmitton



Joined: 04 Aug 2014
Posts: 4

PostPosted: Sun Aug 10, 2014 7:16 am    Post subject: Re: Low RPM on Takeoff??? Reply with quote

Thanks for all the help so far...but no solution yet.

We have checked just about everything everybody suggests but no luck.

Went over the carbs with a magnifying glass - couldn't see any leaks. sprayed it with hair spray just in case to try to seal anything micro small. We even took out and inspected the diaphragms up close.

No obvious intermittent grounds. Engine runs fine most of the time. Mag check drops about 50rpm both sides every time.

Burning through premium gas regularly and diligently checking for water now.

All the rubber connectors into and out of the carbs are in good shape without cracks.

Changed the oil, cleaned or replaced the filters (air, gas and oil) and lubed every other moving part. Even changed to a brand new backup propeller we had and carefully tourqued all the bolts in place.

We have marginally improved - WOT to about 5100 on first application and up to about 5300 after the 800ft run before t/o. (As high as we want in the air).

Having a professional Rotax mechanic take a look but I will be very surprised if he finds anything obvious after our detailed examinations.

Oh well - it still gets off the ground and pulls the plane along (Tecnam P96 Golf) so we still fly! Just wondering where our previous get up and go went?


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John Hauck



Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)

PostPosted: Mon Aug 11, 2014 5:44 pm    Post subject: Low RPM on Takeoff??? Reply with quote

If your throttle levers are spring activated, they may not be pulling the
throttles wide open.

john h
mkIII
Titus, Alabama

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tmitton



Joined: 04 Aug 2014
Posts: 4

PostPosted: Tue Aug 26, 2014 7:55 pm    Post subject: Re: Low RPM on Takeoff??? Reply with quote

Latest update...no change or improvement. Still WOT on ground of 5100 max.

- sent ignition modules back to Rotec in BC and received a 100% good/clean bill of health.
- fixed intermittent ground through one "mag" switch
- cleaned low time plugs
- removed carbs and rubbers again, inspected up close under magnifying glass and replaced them.

One carb seemed to be very slightly "wet" (seemed to be gas versus water condensation) around the o-ring on outlet side rubber boot when disassembled. No obvious cracks or flattened O-ring but new ones on order for replacement.

Next step: carbs going out to experts for inspection while we try new carbs.


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