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Rotax 912ULS Idle speed to high

Art_N412SB

Member
Hello friends,
I am new to the RVs and the Rotax engines. As a matter of face after 50 years of professional aviation this is my first aircraft ownership. I picked a great airplane to buy into. 3 of us own this RV12. I have had 4 flights in it and am in love. :)
So this week finds us doing a 100/200 hour inspection at 400 hours of airframe time. The engine ran flawlessly going into the inspection. But when we rolled it out to do the carb sync, suddenly the idle rpm went to 2500. We are having a tough time figuring out what the difference a day would make.
Let us suppose that we went through all of the obvious things. (which are not obvious to me) and we decided to do an idle speed adjustment. The Rotax maintenance manual 10.5 Idle speed adjustment procedure refers in step 1 to adjusting the idle mixture screw. Is that a factory only adjustment? Do you guys ever adjust the engine idle speed using this screw in addition to the idle speed adjustment screw of the throttle valve?
Thanks a bunch,
Art
 
The engine ran flawlessly going into the inspection. But when we rolled it out to do the carb sync, suddenly the idle rpm went to 2500.

Did you mess with the carbs in any way during the inspection?
 
You can adjust the idle mixture screw, but at the ROTAX course the guy told us most people who mess with it cause more issues. The ROTAX manual tells you to set it by screwing it in and then backing off a set number of turns I think 1 1/2: but I don?t remember for sure. It should have been set at the factory, and unless you changed it I wouldn?t mess with it.
 
You can adjust the idle mixture screw, but at the ROTAX course the guy told us most people who mess with it cause more issues. The ROTAX manual tells you to set it by screwing it in and then backing off a set number of turns I think 1 1/2: but I don?t remember for sure. It should have been set at the factory, and unless you changed it I wouldn?t mess with it.

1 1/2 turns is correct...
 
check the idle stops (the brass stops that you lock in position with the tiny allen key) its easy to have those setup to prevent you getting to a low enough throttle cable position.
 
Thanks, I will watch the video. I looked at one on Youtube of a complete overhaul of the carburetor. That was pretty interesting. Took about an hour though.
 
idle speed

I would check the throttle levers are returning to their stops
also try removing the filters they may have too much oil changing the air flow
 
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I would check the throttle cable is returning properly and has a little slack at idle setting.

At idle, the throttle cables are under tension, right?

I think I made the same mistake here and was corrected. My excuse was years with the same basic carbs on motorcycles where the throttle springs pull the lever to idle if a cable breaks. On the ROTAX it?s the opposite - if a cable breaks the spring pulls it to full throttle.
 
How old is the 912?

Just had a 5-year old 912 on a E-LSA at my airport that had same or similar issue. (3,000 RPM idle)

Problem was gaskets in the carbs. One had the cork gasket around the outside of the float broken at the idle mixture jet and there were some O-Rings found were not correctly seated and one O-Ring was broken.

Replacement parts were better quality with a replacement cork gasket now a conventional gasket. Overhaul kit for both carbs was around $150.
 
Art ... you have received good suggestions so far. I would also like to add take a very close look at the throttle return springs attached to the throttle arm ... they have a propensity to break over time.
 
I forgot to mention that I was with those units at Can Tho as Well Ric.

Art....my buddy (Joe Conley, who just bought half interest in my-12) and I both flew chinooks ( I, Innkeepers, he, Hillclimbers) for the first half of our tours; then ran the 307th TOC-Phantom, the second half. We were a little ahead of your time in Country.
 
It sounds like something common to both carbs. Throttle and cable, etc seem like something to double check.
 
We have not checked the floats as yet, so the bowels have not been removed. I am liking the idea of some kind of induction leak.
 
It sounds like something common to both carbs. Throttle and cable, etc seem like something to double check.

No, can quite easily be just one carb. Take a look at both EGT's and see if they are close to same temp. If not, higher temp EGT is imbalanced carb and doing most of the "work".
 
Art - don't know if you managed to fix this yet, but there is an excellent A&P IA, in Arlington, WA who has a ton of RV12 experience and is a Authorized Rotax repair center. His name is Jim Scott and his company is Aircore Aviation.
 
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