Air bubbles in needle valve on OS 1.60FX
09-25-2005, 08:16 PM
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Air bubbles in needle valve on OS 1.60FX
Hey Bax,
I've got an OS 1.60FX with a Perry VP30 pump that I've had for a few years, and last year it developed a habit of going lean in flight. Since the crankcase pressure line to the pump was dry-rotted, I figured that was the problem and replaced it. Never got a chance to fly it again until recently.
First flight was fine. Second flight... went lean again. This time I pulled the cowl, and found air bubbles in the line between the remote needle valve and carb. From what I can see, the pump actually SAVED the engine and plane, keeping enough fuel going to the carb to keep the engine running at partial power.
It's leaking air somewhere, but I can't tell where. It doesn't seem to be leaking in around the needle, because the fuel-tubing trick didn't help. It's solid fuel from the pump to the needle, but air bubbles galore from the needle to the carb.
I've got an OS 1.60FX with a Perry VP30 pump that I've had for a few years, and last year it developed a habit of going lean in flight. Since the crankcase pressure line to the pump was dry-rotted, I figured that was the problem and replaced it. Never got a chance to fly it again until recently.
First flight was fine. Second flight... went lean again. This time I pulled the cowl, and found air bubbles in the line between the remote needle valve and carb. From what I can see, the pump actually SAVED the engine and plane, keeping enough fuel going to the carb to keep the engine running at partial power.
It's leaking air somewhere, but I can't tell where. It doesn't seem to be leaking in around the needle, because the fuel-tubing trick didn't help. It's solid fuel from the pump to the needle, but air bubbles galore from the needle to the carb.
09-26-2005, 09:56 AM
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RE: Air bubbles in needle valve on OS 1.60FX
Air bubbles forming at the needle valve can be from a variety of sources. One is, of course, a leaking needle. However, on a pumped engine is isn't really likely due to the pump's pressure. An air leak at the needle would be cause by carburetor suction, no positive fuel pressure. If the needle assembly leaked on a pumped engine, you'll get fuel coming OUT, not air going IN.
Another possible source is heat. If the needle assembly gets hot enough, it could cause fuel vaporization.
Vibration can agitate the fuel in the tank enough to make micro-bubbles in the fuel. The needle provides a stagnation point where the micro-bubbles can join and make visible bubbles. In either case, the engine will trend lean.
If the engine's been sitting a while, long enough for fuel line to deteriorate, then suspect the pump. Your pump is a diaphragm pump operated off of crankcase pressure. If the diaphragm is bad, then gasses from the crankcase could make their way into the fuel system.
Several things need to be checked. If you can disconnect the pump system and run the engine on straight muffler pressure, try it. You will find if the pump's the source.
Another possible source is heat. If the needle assembly gets hot enough, it could cause fuel vaporization.
Vibration can agitate the fuel in the tank enough to make micro-bubbles in the fuel. The needle provides a stagnation point where the micro-bubbles can join and make visible bubbles. In either case, the engine will trend lean.
If the engine's been sitting a while, long enough for fuel line to deteriorate, then suspect the pump. Your pump is a diaphragm pump operated off of crankcase pressure. If the diaphragm is bad, then gasses from the crankcase could make their way into the fuel system.
Several things need to be checked. If you can disconnect the pump system and run the engine on straight muffler pressure, try it. You will find if the pump's the source.
09-28-2005, 04:47 PM
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RE: Air bubbles in needle valve on OS 1.60FX
It wasn't fuel line that deteriorated. It was the hard plastic crankcase vent line to the pump actuator. Apparently it's a problem because Conley is now shipping their pumps with Tygon tubing.
The pump seems fine on the ground. Wobbling the prop back and forth by hand causes it to pump LOTS of fuel. Too bad I can't run it with muffler pressure only... No muffler pressure from this darned Pitts muffler. I have come to hate Pitts mufflers... way too much exit area, and there's no plug on the face of this earth that will stay in the muffler pipe on an engine this large.
All I know is that air is getting in at that needle valve somehow. Maybe it's pumping fuel out too while it's running? I dunno.
The pump seems fine on the ground. Wobbling the prop back and forth by hand causes it to pump LOTS of fuel. Too bad I can't run it with muffler pressure only... No muffler pressure from this darned Pitts muffler. I have come to hate Pitts mufflers... way too much exit area, and there's no plug on the face of this earth that will stay in the muffler pipe on an engine this large.
All I know is that air is getting in at that needle valve somehow. Maybe it's pumping fuel out too while it's running? I dunno.
