912s Engine Problem Help

[brannan78]

We are having a curious issue with our 912s (Speedster IV) and was hoping you might be able to lend an idea.

On a three occasions after takeoff during or soon after initial climb-out the engine acts as though it’s failing (decreasing in power and RPM) with almost a sputtering sound. At first we thought it might be too lean but the plugs look okay. Thought about maybe bad diaphragms in the carbs but would like to know if others have found this.

In two cases powering back to less than 4,000 RPM helped the issue. It does it for about 30 seconds soon after it runs fine for the remainder of the day, and the issue doesn’t arise for weeks and multiple flights after.

On two of the occasions it happened in the first minute of flight and not again. On the third occasion it happened in the first minute on the first three takeoffs and then not again.

Any ideas?

[ClickClickBoom]

Carb ice?
Not a popular concept, but after prolonged idling(like waiting for engines to come up to temp), carb ice is a very real potential. All it takes is for the right combination of temp and humidity, no matter how improbable, the possibility is there.

eric

[Av8r3400]

Hot or cold? If hot, What fuel? If auto gas do you have a return line to prevent vapor lock?

[brannan78]

Hot or cold? If hot, What fuel? If auto gas do you have a return line to prevent vapor lock?

You mean outside temp? If so outside temp I'd guess to be between 70F and 90F.

If you mean use, three occasions were from cold starts and small amount of run-up time.

Using 100LL at present time.

[ClickClickBoom]

You mean outside temp? If so outside temp I'd guess to be between 70F and 90F.

If you mean use, three occasions were from cold starts and small amount of run-up time.

Using 100LL at present time.

The question is what was the relative humidity?
The common consensus seems to be that the 912 is immune to ice since they draw air from deep in the cowl, but the reality is the location may delay/lessen the possibility, but it cannot prevent it entirely. .
eric

[Dave S]

Brannan,

I'll throw this out in the blind - obviously a person cannot truly troubleshoot a situation they do not have their hands on directly so take it as just an idea.

Do you know what your fuel flow is? i.e. the amount of fuel the system is capable of delivering to the carburetors. Fuel flow potential/delivery rate must be more than what the engine actually needs/burns

Do you have a fuel pressure gauge?

The reason I ask is two of the symptoms you describe may or can be indicative of an intermittent fuel delivery issue.

1) Decreasing power and RPM on takeoff following maximum throttle
2) The fact that reducing the throttle permits the engine to run smoother.......throttle reduction could be allowing the fuel delivery to catch up with the demand.

If you have a fuel pressure gauge - this possibility could easily be confirmed or refuted.

Sincerely,

Dave S
KF7 Trigear
912ULS Warp

[ClickClickBoom]

You mean outside temp? If so outside temp I'd guess to be between 70F and 90F.

If you mean use, three occasions were from cold starts and small amount of run-up time.

Using 100LL at present time.

Cold engine, OAT 70-90F, depending on the relative humidity, that is the sweet zone for carb ice. The cold engine equation means the intake runners had not had time to have heat transfer from the cylinder heads. Fuel starvation could mimic these symptoms, but would likely reoccur at times regardless of the situation.
Interesting at the very least! Occam's Razor.....
eric

[ClickClickBoom]

the issue doesn’t arise for weeks and multiple flights after.

On two of the occasions it happened in the first minute of flight and not again. On the third occasion it happened in the first minute on the first three takeoffs and then not again.

Any ideas?

If you were flying from or near a powered airport the temp/dewpoint spread might be available online.

[Av8r3400]

You mean outside temp? If so outside temp I'd guess to be between 70F and 90F.

If you mean use, three occasions were from cold starts and small amount of run-up time.

Using 100LL at present time.

Yes, I'm talking OAT. (My first reply was from my phone so typing was not easy...)

Heat in the cowl can be an issue. The design of the fuel pump and lines often leads to vapor lock or partial lock. This is one of the reasons that Rotax is pushing the return line very hard. The return line starts from the pressure line, through a .014" orifice back to the (header) tank. This return line offers enough flow to keep the fuel from boiling (locking) in the line.

The second place I would look to is fuel flow. There are some known issues with the mechanical pumps on the 912 motors (and the can be intermittent). There have been several updates in recent years. If your fuel pump is weak, this could be your problem. Do you have a supplemental (electric) pump? Would you consider adding one? Example

I can't agree on the immediate jump to carb ice. A Model IV, round cowl with a 912S is almost impervious to ice. If you were launching into hard IMC with dew points perfect, maybe you could get some ice, but even then it is pretty unlikely. I've flown in freezing fog situations while on skis and never got carb ice (some airframe ice, but that's a different problem). The design of the Bing CV carb and it's location in the cowl does not lend to ice formation.

The only time I have ever heard of carb ice on a 912/Kitfox was on an installation that utilized the carb-heat, air box, exhaust-muff setup. The only reason for this was the location of the intake, taking cold air from the front of the cowl right behind the prop, when the heat was not engaged. This is the main reason I took this system off my plane.


Very nice looking plane, BTW!!

[kmach]

Hi,
It sounds like a fuel flow problem to me.
I would check everything in the system from the wing tanks to the carbs. something could be blocking the flow intermittently. any recent work done ? sometimes hose clamps can be tightened and restrict flow.

I really doubt carb ice, though in theory is possible.

Hope you find the problem.