912ULS-RPM drop down

by moshe rudko in 912 / 914 Technical Questions 6 years ago

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Re: 912ULS-RPM drop down

by Roger Lee » 6 years ago

Roger Lee

126 5.13k 4.58k

Hi Moshe,
Thanks for the additional info. It does sound like a fuel issue. Is this issue repeatable on the ground?
When you give it throttle is it a quick addition of throttle or a little slower smooth addition?
Just to rule this out I would drop the carb bowls and weigh the floats. You have to start somewhere and start ruling out common causes. This may not be the problem since it is affecting all of you. This may be an installation issues since this is affecting 3-4 people the same way. The max weight when each SET is weighed is 7 grams. The average I see is 6.1 - 6.5 grams per set. Take each set out and dry them off then weigh them. If they are okay then I would put a set of carb sync GAUGES on and check the sync. The gauges are better than an electronic device because it's easier to diagnose problems and see changes as you use the throttle. 5C (41F) - 12C (54F) is not really cold weather and that shouldn't be affecting all of you. Do you have at least 7.9mm (5/16") hose from the fuel tank to the fuel pump? Out of curiosity what is your plug gap? Should be between .023 - .027.

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

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Re: 912ULS-RPM drop down

by moshe rudko » 6 years ago

moshe rudko

5 20 0

Thank you Roger for your guidelines. I will have some checks and reply soon. Also will put more attention about the engine's performance on the ground versus airborne playing with the throttle in a different situations (fast/slow acceleration, etc.
Appreciate your support.

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Re: 912ULS-RPM drop down

by moshe rudko » 6 years ago

moshe rudko

5 20 0

Hi,
So it took me awhile to fulfilled Roger's recommendations.

Bottom line - I am a desperate pilot!

The carbs were re-tuned and balanced and the bowls are of the new type (3 marks) and weigh as by the book.
We checked the hoses and in fact the entire engine's components (visually) and found nothing that may cause it.

The weather become warmer now (spring time) and the ambient temp is around 10-20 Celsius. The humidity I presume isn't too high.

And yet, Take off and after 30 minutes flights maintaining varied level flights the engine starts to lose power for a few hundredths of sec'. The RPM indicator shown a swift drop and then recovered.

I can say that sometimes after flight I see a condensation outside around the carbs body till it gets dry.

IT FREAK ME OUT.

It is a brand new engine with 44HRS only. And an the engine was checked by Rotax authorized technician.

Everyone blames the carb-icing or the new generation fuel composed of more ethanol then use to be.

PS: There are about 5 engines experiencing the same symptoms on a different aircrafts.

HELP please HELP

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Re: 912ULS-RPM drop down

by Roger Lee » 6 years ago

Roger Lee

126 5.13k 4.58k

It is concerning that you say 5 engines are all having the same issue. So the normal individual checks may not apply as it might to just an individual. So it may be time to look for a common denominator. If you think it may be fuel related then dump the auto fuel and try some 100LL. This will at least rule fuel in or out. Fuel in your specific area could be an issue. Are all 5 engines using auto fuel or 100LL or are they a mixed use between the owners?

"I can say that sometimes after flight I see a condensation outside around the carbs body till it gets dry."

This may be the smoking gun here. Condensation on the carbs. You're flying around 15C (59F) and it is possible with some humidity that you have a little carb ice, but hard to imagine all 5 are having the same thing, but anything can be possible. Does anyone have heated carbs or carb heat? Are all engine open air engine's like yours or are some closed cowl? Ask around and find out about everyone's fuel use, flying conditions and type of engine mount (open vs cowled, carb heat)

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

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Re: 912ULS-RPM drop down

by moshe rudko » 6 years ago

moshe rudko

5 20 0

They are all open engines as mine. The average cruise altitude is between 500-1000ft above the sea level.
We all using the same auto fuel grade (95 unleaded octane(RON). Guess it similar to the American 85 octane) but refuel in a different gas stations (our aircraft is mobile, on a trailer so we refuel it in every auto gas station). The flight condition is more or less the same even thought we departs from a different airfields/area.
No one is using carb heat system in this type of aircraft but we are serious considers to install the system (the electric type system) to clarify if it helps.
BTW, it happens only on the 912ULS. The 912UL and the 582 aren't infected by this symptom at all.
I can try refuel 98 unleaded octane but for that, do I need to completely dry the 95 octane of the tank and the engine? Moreover, do we have to change the engine setup due to this? Is this much different between the 95 and the 98 grades (RON)?

And last but not least, Is this official and supported by Rotax to install carb heating system? liability, performance, etc.

It isn't common to find 100LL unless if your base is at a commercial aviation plant. We usually take-off from any field or country side roads with no access to thus commercial airfield.
Thank you Roger for helping us

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912ULS-RPM drop down

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