914UL carburettor gremlins

by Erik Gunneland Dahbäck in 912 / 914 Technical Questions 6 years ago

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Re: 914UL carburettor gremlins

by Erik Gunneland Dahbäck » 6 years ago

Erik Gunneland Dahbäck

2 6 1

Alright, torque wrench for the bottom bolt is duly noted. :)

However, does misaligned float rods (ie. the floats not moving freely) really explain either the initial problem I experienced or the last one reported by the mechanic during recent test runs. That is, can they really cause major loss of power when adding power?

To recap, initial problems: Rough running and sudden, sharp RPM-drop when trying to increase RPM above approx 4500 RPM. Current problems: The same, abeit even more brutal and as early as above 3000 RPM. Doesn't this seem more like a lack of fuel than an overflow?

During the last few days, the mechanic also tried testing fuel- and airbox pressure as per the "Fuel Pressure Checking and Adjustment - 914"-video.

It turned out to be hard to measure the relative values, likely due to the engine not being able to run above 3000 RPM (and for the additional reason below). However, the absolute values were constant, around either 5 or 8 PSI depending on whether one or both fuel pumps were running. This was also not affected by running the engine or having it stopped. What gives...?

Also, strangely, he wasn't able to get any meaningful readings at all regarding airbox pressure, despite changing guages and verifying that they were working.

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Re: 914UL carburettor gremlins

by Tony Armour » 6 years ago

Tony Armour

0 17 28

Your issues do sound similar to mine when I had the distorted bowl issue. These bowls (as I'm sure y'all know) are pressurized with boost air. The distorted bowls if not sealing well at the gasket will leak air/boost and the engine will run bad.
My issue: I was just flying along (5,100 rpm) (after a 2 hour leg and fuel stop then another hour) and it started running rough. If giving more throttle it would get worse. I had to pull back on the throttle to smooth it out. I ended up doing a precautionary landing (probably would have been forced down if I had kept flying) Before landing I was around 4,000 rpm or less to keep it smooth. Spent the night, bowls off and on, finally cleared up. I flew another three hours (2 days on this long trip) with no issue and then it started again on a morning run up. I ended up using my waste gate switch and flying the hour+ back to Lockwood Aviation in Florida and we put two new bowls on. No issues after that.
I m having a different issue now that most likely is carb related. That's why I ended up here. These things are higher maintenance than a women !!! :-)
Try a run-up with the waste gate open (via the waste gate switch, what ever it's called) and I bet you can run it to 5,000 rpm+ smoothly.
If your bowls and gaskets have no issue then I would search for other leaks from the bowl.
Does anyone know if the needle valve comes in a carb kit ?

Thank you said by: Capt. Siva,

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Re: 914UL carburettor gremlins

by Erik Gunneland Dahbäck » 6 years ago

Erik Gunneland Dahbäck

2 6 1

Interesting!

Now, let me think, the waste gate default position is off, right, so if I have a switch that disables the TCU, that should keep the wastegate open? (either that or I don't turn on the avionics switch). So would the air leak issue be solely connected to the turbo? I suppose that also means it is a 914-only issue?

If there is an issue with the float bowls: would it be a matter of plastic or elastic deformation, ie. should the bowls return to normal shape when the bolt is released and tightened using the correct torque?

We got a needle at least when we ordered the "MAINTENANCE SET CARBURETOR".

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Re: 914UL carburettor gremlins

by Tony Armour » 6 years ago

Tony Armour

0 17 28

I guess the TCU switch basically turns the waste gate actuator off. I would turn my master switch on and when the waste gate cable did its initial (test) cycle I would turn the TCU switch off when the waste gate was in the open position. It stayed open and thats the way I flew it. You will be at 75 horsepower though so plan for plenty of runway if you are taking off. In the air once at altitude it was fine (was 1,000 msl, beware if you live in high elevations)
Since the bowls are metal, they will not return to shape. Lockwood mech took one look after pulling off and said "here is your problem. In the field or for testing the rods might could be bent back straight or what I would try first is pressing or hammering the bottom of bowl down (opposite of what over torque would cause) and see if that pulled the rods back straight. Still have to make sure id pressing or hammering that the gasket surface doesn't deform or that starts the other issue. I worry about bending the rods...will they bend or break ? I have my old bowls that I guess I could play with.

The two issues with bowls:
1. Leaking around the gasket and the bowl is no longer pressurizing to overcome boost in intake.
2. My issue, the bowls were deformed from over tightening and the floats were hanging/rubbing the side of the bowl. I could visually see the rub spots on the sides of the bowls and could also see that the bowl rods that the floats ride on were bent outward at the top.

For gasket issues, I was also told to put 912 bowl clips on along with the bowl bolt to help spread the load. I am going to look to see if I think that would also help too.

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Re: 914UL carburettor gremlins

by Tony Armour » 5 years ago

Tony Armour

0 17 28

Any updates ? I think I have determined that my high EGT issue is not carb related since it's one cylinder only. Carb overhaul was straight forward though.

I have a buddy with a 914 that was having rough running issues when low rpm like on short final. His issue was distorted carb bowls. He straightened the bowl guides up and no more issue. I have not checked back to see if it was still cured.

Thank you said by: Capt. Siva,

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914UL carburettor gremlins

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