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Old 01-23-2011, 02:38 PM   #1
Av8r3400
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Default 912 carb setting

Lately I've noticed a bit of rough running in the mid-range rpm band on my 912UL (Throttled back to 3500-4500 rpm). It has been most noticeable during the cold of the winter.

Reciently I synced the carbs so I know that was not the issue. In talking to one of my friends who is a Rotax guy, he concurred with my assessment that it was running lean.

To fix this mid range issue, we adjusted the needle position from the second (from the top) position to the third. I was amazed at how smooth the engine now runs in comparison.

I just thought I'd pass this on.
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Old 01-23-2011, 05:10 PM   #2
kitfox2009
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Smile Re: 912 carb setting

Hello Dan

I am also wondering about my 912UL running too lean in the cooler temps. It seems to act in a similar fashion when on base and final with the RPMs at about 3000 or less. Just a slight shutter or roughness. Also I noticed the EGTs a bit higher. Does this not also indicate too lean mixture? Any comments would be appreciated!

Cheers

Don

Vixen C-FOXK 912UL IVO IFA

It was +10C here today so I had to remove my recently added "sliding radiator shutters". Great day for flying,though.
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Old 01-23-2011, 05:43 PM   #3
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Default Re: 912 carb setting

Exactly the symptoms that I was getting. When I would throttle back on downwind and turning to base, I would get that rough running.

This adjustment fixed the issue.
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Old 01-23-2011, 05:49 PM   #4
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Default Re: 912 carb setting

I have have the same issue and so does John, my hangar parnter (model IV speedster). Sometimes we have wondered if our engines will quit on final. I have noticed it more on days with high humidity, like just before and just after a snow storm. Low humidity days do not seem to affect it. Do you think an adjustment back to the original needle position is going to be required or desired in warmer dryer weather?
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Old 01-23-2011, 05:57 PM   #5
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Default Re: 912 carb setting

Rotax 912, Bing 64... Jet Needle Clip position 3 is normal.. 3rd from the top of the needle, 1 up from the bottom towards the tip. Lowering the clip on the needle raises the needle and riches the mixture.

My plane came from 5000' MSL to sea level. Ran just like your problems. Lowered clips to raise needle from 2 to 3, rebalanced carbs, smiles all around.


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Originally Posted by Av8r3400 View Post
Exactly the symptoms that I was getting. When I would throttle back on downwind and turning to base, I would get that rough running.

This adjustment fixed the issue.
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Old 01-23-2011, 06:35 PM   #6
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Default Re: 912 carb setting

Quote:
Originally Posted by SkySteve View Post
I have have the same issue and so does John, my hangar parnter (model IV speedster). Sometimes we have wondered if our engines will quit on final. I have noticed it more on days with high humidity, like just before and just after a snow storm. Low humidity days do not seem to affect it. Do you think an adjustment back to the original needle position is going to be required or desired in warmer dryer weather?
Can't hurt to try, Steve. It turned out to be very easy to do. Unhook the enrichener spring and remove the two phillips screws on the top of the carb, remove the slide piston, remove the regular screw in the top of the piston to change the needle position, reassemble. There was no reason to unhook the cables, so no carb syncing needed.

Easy, peasy...
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Old 04-10-2011, 01:50 PM   #7
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Default Re: 912 carb setting

Got to ask how many of you are using carb heat on those decents, carb heat will richen the mixture while any carb ice will cause the carb to lean the mixture.

Safe Flying
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Old 04-26-2011, 09:43 AM   #8
kitfox2009
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Default Re: 912 carb setting

Hello All

Regarding the decent and carb ice possibility. I have the small hot water manifolds that mount directly on the carb throats. They are on continuously therefore prevent any ice formation automatically. Seems to work well.

Does ANYONE have their carb vent tubes connected into their air filters. I had a fire recently and I believe it may have been a result of some fuel spiting out of the vent tube onto the exhaust. Roger Lee (matronics) suggested possibly routing the tubes into the filter.

Anyone care to provide experience with this??

Don
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Old 04-26-2011, 11:05 AM   #9
Dave S
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Default Re: 912 carb setting

Hi Don,

Those of us who have the airbox find that there is a nipple on each side of the airbox designed to receive/connect to the carb/bowl vents. If some fuel burped back - it would then go into the air box chamber.

Also thanks all for the discussion on the carb adjustment & slightly rough running in the range indicated - I do have experiences very similar to Dan - and it seems seasonally/temp connected - not bad but noticable.

Dave S
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Old 05-09-2011, 10:24 PM   #10
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Default Re: 912 carb setting

I did my 200 hour service today with a Rotax repairman. Most of the work was simple and I had actually done a lot of it last fall during my annual but I wanted to see if I could learn a few things from him.

I had noted roughness in my engine at different rpms and it seemed to be worse at about 3000. We noted that a few plugs were darker than normal implying rich running. We dropped the carb bowls and found that the floats were riding a little too high causing the fuel level down there to be above normal. They were adjusted and things really were smoother before any sychronizing was done. Also, my plug gaps were a tad too large and they were remedied. I am surprised at how much smoother it is through the whole range. I was thinking a needle position change was in order but not the case.
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