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Rpm RISE with carb heat

  • jabkwab
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7 months 1 day ago #18311 by jabkwab
Rpm RISE with carb heat was created by jabkwab
Hello all,

I have been experiencing a strange thing on my 912ul powered Kitfox.
When pulling carb heat with the engine running at any speed above 3000 rpm the rpm increases with about 150 rpm. Very opposite of what could be expected of course.

Running without the air filter does not change this behaviour, so an air blockage is not likely in my opinion.
Main jets, needles and other carburetor parts are looking fine
The spark plugs indicate a normal mixture.
The carburetors are set up according to the manual and do require only minimal adjustment to balance, which in my opinion indicates that there is no problem with a single carb.

The engine seems to run slightly rough when accelerating through about 3000 rpm with cold air. With carb heat on acceleration is absolutely smooth.

I do have problems getting the engine to proper operating temperature, always operating just above recommended minimum cht and oil temp.
Could this be the cause of this effect?

I have had this issue for a couple of years now, and to me it does not seem to be a threat to flight safety. It does irritate me however that I do not know the cause of this effect.
On top of that it makes me wonder if the engine is lacking power in general, since my Kitfox seems to be flying on the slow side....

I would really appreciate any input from anyone with more knowledge then me!

Jan

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6 months 1 week ago #18510 by S19Builder
Replied by S19Builder on topic Rpm RISE with carb heat
I can't claim to have more knowledge than you, but I experienced a similar phenomenon when my carbs were pretty far out of sync. Once re-synced, the carb heat performed as expected.
Rich

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6 months 1 week ago #18512 by Bill Hertzel
Replied by Bill Hertzel on topic Rpm RISE with carb heat
It sounds like the mixture is slightly rich but not excessively.
Cold Winter air will richen the mixture, even more, causing the stumble.
Carb Heat (Summer Air) will lean the mixture smoothing things out.

The slight excess of fuel will carry more heat out of the cylinders, causing the lower CHT and Oil Temp.

Confirm that the Fuel Enrichment Circuit (Choke) is closing all the way.

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6 months 6 days ago #18520 by Starfighter
Replied by Starfighter on topic Rpm RISE with carb heat
Of course carburator icing with temp below 10 deg Celcius and visibility below 1nm could be expected.
I don's expect this with high RPM above 4000RPM

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5 months 2 weeks ago #18716 by jabkwab
Replied by jabkwab on topic Rpm RISE with carb heat
The chokes do close all the way, and the carbs are synced.
When advancing the throttle I get a rough spot between 3200 and 3500 rpm, that smoothes right out when applying carb heat.
As mentioned, application of carb heat gives me about 150 rpm extra. Also at full throttle, so I don't really suspect carb ice.
I was always under the impression that carb heat enrichens the mixture (hot air, less dense).
Therefore suspected a lean mixture is the problem.
I can get access to a CO meter, does anybody know the values published by Rotax for the 912ul?

Or maybe I am missing something here?

Jan

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5 months 3 days ago #18850 by jabkwab
Replied by jabkwab on topic Rpm RISE with carb heat
Case seems to be closed, I found a choke valve not properly seating due to a burr on the shaft.
Replacing the choke valve solved the issue.

Jan
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