912 engine cough in humid conditions

by Steven Miller in 912 / 914 Technical Questions 11 years ago

This summer I installed a 912ULS on an Aventura II. I had the same engine installed for four years on a QS GT-500. Since the install on the Aventura II, the engine will sometimes "cough" under more humid conditions, losing 50-75 rpms and then immediately recovering. Sometimes it will only happen once or twice during an hour long flight, sometimes more frequently. When the air is dryer it doesn't happen at all. I'm thinking perhaps some kind of carb icing? I'm somewhat baffled as to why it would start doing this with the new install. Any thoughts as to what may be causing this or a remedy would be greatly appreciated.

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Re: 912 engine cough in humid conditions

by Alan Arnold » 11 years ago

Alan Arnold

20 188 145

Steve,

Do you leave your fuel tanks empty between flights? If so, you might get a drop or two of water into the fuel supplying the engine. That could cause the cough.

Carb icing should only occur under part throttle operation. Are you cruising at around 5200 rpm which is the Rotax recommendation?

Alan

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Re: 912 engine cough in humid conditions

by Steven Miller » 11 years ago

Steven Miller

2 15 0

I do not empty my fuel tanks, and so far the engine runs fine when I fly in relatively dry air. If there was water in the fuel, I assume it would show up regardless of the conditions I was flying in. I'm at about 5260 at takeoff and cruise between 4800-5000. I probably need to adjust my prop (3-blade warpdrive, 72") I also had the thought that my problem may be ignition related. The rpm drop and recovery is very quick--a second or less--and is about the same drop as I get with a mag check. Could moisture exacerbate a failing ignition, or cause it to momentarily fail? The other possibility I considered is a simple change of air pressure on the prop. The Aventura is a pusher, so there is lots going on in front of the prop in terms of how air is directed. I just don't think it would explain a 100 rpm drop.

Steve

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Re: 912 engine cough in humid conditions

by Roger Lee » 11 years ago

Roger Lee

125 5.12k 4.53k

Hi Steve,

What type of fuel are you using? Humidity alone should not cause this. A small slug of water going through the system could, but as you stated is usually a very self limiting event. Have you pulled the carb bowls off yet to take a look? I would highly recommend that you do this. It does sound a fuel issue, but until we rule things out one by one we can't be sure. When was the last time you did a carb sync and done with gauges and not an electronic device?

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: 912 engine cough in humid conditions

by Steven Miller » 11 years ago

Steven Miller

2 15 0

Thanks again Alan for taking the time to address this. I'm using non-ethanol auto premium, usually Shell, which I run through a canister type filter. I did check the carb bowls and they were clean. The engine has not been mechanically synched for at least three years. I do have the kit with gauges but the engine has always run very well so I never used it. Not sure it matters but I do have about a 200 degree difference on EGTS between 4200-4800, otherwise, they are close.

Just got back from a 1.7 hour flight. Kind of hazy and I did experience the intermittent rpm drops (I was at 2500 ft.) I then tried running one mag at a time starting at 4800 rpms and they reacted pretty much the same (initial 180-200 rpm drop more or less, then same type of intermittent drops). Then went above the thermocline (7800 ft) for about 20 min. up to the cool dry air and the rpms were rock solid--no drops. So I take it my plan at this point should be to adjust the prop to get proper rpms at take off and cruise; and then do the rotax carb synch procedure? Any other thoughts at this point? I think I can rule out water in the fuel.

Steve

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Re: 912 engine cough in humid conditions

by Steven Miller » 11 years ago

Steven Miller

2 15 0

Sorry, I mean Roger!

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912 engine cough in humid conditions

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