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drop of rpm on take off

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georg.stiller(at)arcor.de
Guest

Posted: Tue Jan 04, 2011 2:34 pm Post subject: drop of rpm on take off

Hi all,

I just subscribed to the list. What a great forum!

I have trouble with my 80 hp 912. Maybe someone has a good idea and can give me some help.

Background: 

912 with 150 hrs. in a Kitfox, no troubles so far, 98 octane auto fuel, alcohol test negative, aircraft parked in heated hangar, properly drained and engine run up, fuel always filtrated, filter visually clean, carbs freshly pneumatically balanced, mag drop ca. 200/300 rpm (has recently deteriorated).

What happened: 

Very cold outside temperature, about -15 centigrade. Max. ground rpm ca. 5000 rpm (should be 5100, has deteriorated somewhat - under cold conditions?), acceleration and take off normal, a couple of seconds after take off sudden drop of rpm to ca. 4500-4600 rpm. After another couple of seconds and the engine catches up and returns to 5000 rpm. At least, so far it always did. No problems in cruise with 4300 rpm.

I usually take off and cruise also under these cold conditions without carb heat. I tried a take off with carb heat and a couple of seconds after take off: rpm drop as obove!

All this happened four or five times.

Next I plan to change the spark plugs, although they still look good and the manual says change after 200 hrs.

Any help would be highly appreciated!

Thanks, Georg

Kitfox, 912 80 hp

Germany

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simon(at)design24.org.uk
Guest

Posted: Tue Jan 04, 2011 3:55 pm Post subject: drop of rpm on take off

Georg,



I am new to this forum and have a Pulsar with an early 912. My aircraft is

low wing and has a boost pump as the fuel tanks are below the engine. Is it

possible that your problem may be due to low fuel pressure due to the angle

of incidence at the point of takeoff in the kitfox? (very nose high) The

standard Rotax pump can only suck so hard and may be o0n the limit without

help. Fuel boost pumps are inexpensive and easy to fit and i use mine always

for take-off, landing and fuel tank switching.



Good luck!



Simon, UK Pulsar XP   G-BUZB

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ruruny(at)aol.com
Guest

Posted: Tue Jan 04, 2011 6:33 pm Post subject: drop of rpm on take off

Carb float bowl vents run to bottom of firewall?

   

   [quote][b]

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moosepileit

Joined: 10 Dec 2007
Posts: 26
Location: USA

Posted: Wed Jan 05, 2011 8:10 pm Post subject: Re: drop of rpm on take off

I think the air may be too cold for your carb needle setting.  30 degrees C below standard needs about as much extra fuel as when I moved my plane from 5000' MSL to sea level.



My issues were LESS severe than yours.  Setting the carb needle clips down from #2 to #3 position (3 is normal) fixed the issue nicely.



Hopefully someone like Roger Lee can help you out.



I had sync'd my carbs just before finally isolating my issue, after moving the clips the carbs needed only slight rebalance, then it was smiles all around.

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Roger Lee

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

Posted: Thu Jan 06, 2011 6:11 am Post subject: Re: drop of rpm on take off

I agree. I think he is running just a little lean so long as the mag drops are fairly equal. Drop the clip one notch from position 3 to 4. Normal clip position is in number 3.  Position one is at the top. This will richen the mixture some for your cold dense air. Later when it warms for the summer you can switch back. Double check the carb balance after you switch. Let us know the results.

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_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

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