Engine (Carburator ?) Problems

[PH-SCP]

Hi fellow RV'tors,

Yesterday, after completing the checklist and making sure all was in the Green, I lined up and applied full power with my O-320-D2J powered RV-9. The engine immediately quit on the runway...
Managed to start it again (with much effort) and taxi around to the holding point. Re-did all checks, lined up, applied power and the engine died again.
All of this with the aux fuel pump on..

On the taxiway I did checks and with the aux fuel pump "off", the engine runs just fine. Switching the pump on causes bad running at low RPM and a total quit at high RPM.

The local A&P mechanic told me I have a carb problem, the fuel inside the carb reaches a too high level, specially with the aux fuel pump on. He said I could fly it as long as the Aux Fuel Pump remained off. I declined and want to solve the situation first.

Engine: O-320-D2J
Carb: Marvel-Schebler P/N 10-5135
Temp: 28 C
Elev: 1200 ft

Before investing in a new carb, I would like to hear from one of you experts if this indeed sound like a carb problem or should I look elsewhere... ?

All the best !

[DanH]

Do you have a fuel pressure gauge?

[mahlon_r]

I agree, you need to know what the fuel pressure is, with the boost pump on, before condemning the carb.
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

[PH-SCP]

Originally Posted by DanH

Do you have a fuel pressure gauge?

No, I don not have a pressure gauge only a fuel flow gauge connected to the Dynon 100/120. This was indicating 0.0 GPH flow with the engine running at low RPM.

I have heard suggestions of vapor lock (I am using 98 octane mogas without alcohol) since it was a hot day but vapor lock would occur at all RPM settings and more likely, would disappear while turning the boost pump on. In my case the engine quit when turning the boost pump on.

Any thoughts ?

[mahlon_r]

Could you have the inlet and outlet of the boost pump reversed??? So that when you turned it on, you were actually pumping fuel away from the engine?
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

[PH-SCP]

Originally Posted by mahlon_r

Could you have the inlet and outlet of the boost pump reversed??? So that when you turned it on, you were actually pumping fuel away from the engine?
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

The airplane has 230 hours and the system worked great until yesterday. Unless it reversed itself by forces of magic, this is not the case.

With the electric boost pump on, the engine kills itself because it's getting too much fuel. The reason why is still unclear to me.

Thursday we shall travel to the plane, now stuck in Germany, with a new carb and a new fuelpump and start troubleshooting...

[ClayR_9A]

I'm just guessing, but it sounds to me that when the float goes up and the "valve" that stops the flow of fuel closes, it isn't closing completely. Maybe some crud or something stuck in there?

Could also be a float that isn't floating anymore. (there was a service bulletin on the floats getting a hole in them and sinking)

[PH-SCP]

An engine expert explained to me that he suspected the engine-driven fuelpump to be at fault. According to his opinion, the engine-driven fuelpump keeps a steady pressure in the line that runs to the carburator. Switching on the aux fuel pump should not change the pressure in this line, it is just there no supply pressure in case the engine-driven fuelpump fails. A set of membranes keep this pressure at a constant level (so I'm told)... In his opinion these membranes failed which many A&P mechanics say is unlikely and hardly ever happens.. The result is that both pumps (engine-driven fuelpump + aux fuel pump) increase the pressure in the line to the carb and over-deliver fuel, causing the fuel chamber to overfill, smothering the engine.

Anyway, to be totally sure we brought a new engine-driven fuelpump and a new carburator to the plane and installed the engine-driven fuelpump first.
We ran the engine in all thinkable RPM/Aux fuel pump on/off-combinations and it ran wonderfully nice. No black smoke, no failures. Fuel flow was back on, even running at idle (before the fuel flow indicated 0.0 GPH when running idle)

Flew the plane back home and haven't had problems since.

So, eventhough an engine-driven fuelpump hardly ever fails, it's not unbreakable !

[Walt]

Hmmm... The standard facet pump is only capable of 4-6 PSI normally so even though it may be working ok now, it still does not make sense that replacing the mechanical would fix it.