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Hotscam 03-07-2017 02:19 PM

Carb ice ?
After over 5 years of happy RV12 flying we seem to have all problems this year.
We have two RV12 and on both we had engine problems and critical emergency landings.

PH-SEP hobbs 600 5 years in use :

We did an extensive annual a month ago and as the 5 years had past we replaced all rubber hoses and components as per Rotax schedule.
We had a Rotax dealer rebuild the carbs and replace the fuel pump both due at 600 hours.
We did a 30 minute testfligth after all the work and all seems perfect.
Today we did a first 45 minute overland without any problems.
On the return fligth however it was very difficult to reach rotation speed untill we noticed the engine did only 3800 rpm but we had to depart as the end of the short grass strop was near. We got up to 300 ft and the engine shaked and ran very rough and we barely managed to go around and land. Pfffff
We called the local Rotax engineer which did a lot of checks but found nothing.
After a succesful testflight we returned to our airport without any problems.
In all cases the engine was ran for a good warm-up before departure. Humidity was quite high and temp 50 F. Both fuel pumps on.
We analysed the Dynon log files and all readings are normal with exception of the two right hand EGT's were 300 deg F lower than left and varying up/down.
The experts concluded short of fuel caused by carb ice !
I was always made to believe as we have no airbox the carbs always get warm air from inside the cowl and therefore heating not required.
I must also say in 850 RV12 Rotax hours we never had ice and flew in all kind of temperatures and humidities.
Also never heard any RV12 owner about this.

What say and are there carb ice exoeriences ?

Jolly 03-07-2017 02:43 PM

Hi jack,

Loosen the carb rubber intake screw and push the carb back out of the socket. This allows the carb to be lifted. Pull the bowl off the carb. I have ten bucks you have some debris in the carb bowl. If you don't do this you may have the issue with reduced rpm at the wrong time again.

bajapilot 03-07-2017 02:50 PM


In the 200 hrs of flying my RV-12 I have yet to experience any carb ice.

However, in my 1100 hrs flying my Rans S-7S I have experienced several instances of verified carb ice on the ground before take off. Never in the air. This has usually happened flying into the back country airports in Idaho where morning temps are frequently in the 30's and 40's and the humidity was high enough to have visible water on the wings and windshield. I've had the 912 ice up badly enough to quit as I was taxing to run up area.

As for verifying it. My Rans does have carb heat which I rarely use. But each time I've had icing, applying carb heat caused the engine to spit and sputter for several seconds and then pick up 200-500 rpm without increasing throttle position. Classic carb icing.

I don't think you'll ever have a problem in flight because of the heat generated at flight power settings but on the ground in cool weather your not going to get much heating inside the cowl even though you might warm the engine to T.O. min temp.

My 2 cents. Bill

Jolly 03-07-2017 03:47 PM

He just did a hose change and carb ice won't knock down the rpm to 3800. Carb debris will.

aerofurb 03-07-2017 04:19 PM

Hi Jack

Even if it turns out not to be ice, for information we fitted this kit on ours:

The advantage being the carbs are kept warm and toasty (ie ice free) permanently and there is no loss of power because you're not running on hot air like conventional muffler-fed carb heat systems.

BobTurner 03-07-2017 04:52 PM

Without knowing anything specific, I just point out that the FAA carb icing chart has high humidity, 50 F as just about the worst condition for conventional carbs and icing.

AirHound 03-07-2017 05:52 PM


Originally Posted by Jolly (Post 1155891)
He just did a hose change and carb ice won't knock down the rpm to 3800. Carb debris will.

Hotscam - Go with Jolly's suggestion and check/verify with your own eyes for debris/hose-change residue which could have made its way.

Driftdown 03-07-2017 08:31 PM

Based upon the initial report, my vote goes to carb debris.
Drop the bowls and inspect!

This is a typical scenario of carb debris following a rubber change.

Hotscam 03-08-2017 06:05 AM

I have been looking at the Skydrive system already, but the space is pretty cramped at the right side due to the oil tank and hoses. But if you get it in..
The beauty is fluid cooling.. Sounds good..

On the other hand I don't believe in icing myself on the RV12 so much.
I never heard anybody who had it sofar in the air.
I was reading old posts from VAN's and the thoughts behind the design without carb heating sound like acceptable to me.
When you would leave the cowl or top of the cowl off it will certainly happen as we experienced when carb tuning without cowl at low temps.

Jolly's suggestion seems most logical to me.
I already lifted the carbs and cleaned the bowls.
I found some dirt, some hair (??) and a drop of water at the side of the failing cylinders. It did not seem a lot but a small particle can easily block the valve needle I suppose. This could never pass the gascolator so it must have been in the new hoses.
The other bowl was clean.
Maybe we should dismantle the fuel lines and flush them, which I maybe should have done in the first place.

Still like to hear anybody who had ice in flight on his 12.

Thanks for your suggestions sofar. Great help

Dpick23 03-12-2017 05:08 PM

carb ice
I have been flying my -12 here in Northern Illinois and Wisconsin for over 200 hrs (at times the temps well below freezing- like this morning when OAT at 5500 was 0 degrees F) and not one incident of carb ice- I love this little engine! You treat it nice- it treats you nice!


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