RPM difficult to control

by Albert Tan in 912 / 914 Technical Questions 6 years ago

Hello!

I've flown the Z701 using 912, and with throttle fully aft, gets about 2000RPM.
I find difficult to control is basically about the first inch of throttle from idle, it's very "jumpy", RPM from 2000 will suddenly jump the 3000, with only a slight movement of the throttle.
This causes very difficult control especially during approach and flare.
At higher settings (about 3500RPM to max) it does "jump" a bit, maybe 200 RPM but thats not too bad.
Best way to describe is sticky response of the engine to throttle movement.

Any ideas?

Cheers!

Re: RPM difficult to control

by Bill Hertzel » 6 years ago

Bill Hertzel

13 1.95k 7.96k

Advancing throttle in the 912 and many aircraft engines is the opposite of most land-based engines.
When you Open the throttle you are giving slack to the throttle cable and the throttle spring at the carburetor is doing the actual opening.
If the cable does not feed out smoothly, the spring will not open the throttle smoothly and may stick and then suddenly release, accompanied by a sudden change in RPM.
If only one side sticks, the carbs will be at different power settings and the engine may run rough.

You should be able to observe this on the ground, engine OFF. Look at each carb.
The carb throttle arm should smoothly follow the movement of the throttle in both directions.
Increasing the throttle is the more sensitive to any binding.
Reducing throttle is brute force movement.

If the throttle cable were to break in your car you would want the engine to default to idle. Wide open would be a disaster situation.
If the throttle cable breaks In an aircraft in flight, have the throttle default to wide open will result in a manageable nuisance.
Going to idle would be much more undesirable.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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Re: RPM difficult to control

by Alan Arnold » 6 years ago

Alan Arnold

20 188 145

In other words, your throttle cables are sticking or binding and not allowing a smooth movement out. Introducing some fairly thick lubricant into the cable housing, not WD40, might help.

Alan

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Re: RPM difficult to control

by Genival Pacheco » 6 years ago

Genival Pacheco

4 87 135

What lub is the better?

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Re: RPM difficult to control

by Kevin Stewart » 6 years ago

Kevin Stewart

3 387 1.16k

My airframe manual says to use engine oil as the lubricant for the throttle cables. Before you do that you might want to make sure that a strand of the inner cable hasn't become detached and is binding inside the outer sheath.

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Re: RPM difficult to control

by Bill Hertzel » 6 years ago

Bill Hertzel

13 1.95k 7.96k

A heavy lubricant is preferred.
40W Engine Oil, 90W Gear Lub, or even a light grease as long as you already have it apart.

If it is binding you might have badly worn or broken strands.
There is probably a good deal of ground metal from the binding of the Cable, so disassembly, cleaning, and inspection are in order.

Unless the damage is trivial, a total cable replacement installed with light grease may save a lot of future grief.

Once you touch the cables, a Carb Sync procedure will be needed.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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RPM difficult to control

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