djlyon
Posts: 1941
Joined: 12/10/2001
From: Castaic,
CA, USA
Status: offline
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My YS53 has bubbles in the fuel line between the regulator and the carburetor when running, from idle to half throttle. At full throttle the line seems to be clear. I don't do anything about it because the engine runs great through out the throttle range and produces great power and besides it is a common condition, which the experts have blessed as normal.
On a RC online thread awhile back it was discussed at length and I presented a far fitched (imo) theory that perhaps the bubbles were vaporized alcohol, the vaporization taking place as the fuel exited the pressure regulator. I didn't really believe this. I thought there was an air leak in the system some where. No body on the thread commented on the theory and I lost interest because my engine and others that I knew of ran just fine.
Now to the OS 91FX, a fine running motor that I've been running for a long time. Good idle, good throttle response and makes really good power. However posts on RC online and magazine reviews have complained about air leaks in the remote needle and discussed what to do to fix them. A few weeks ago I noticed that at low and mid range my engine had bubbles between the needle valve and the carburetor. I ignored it because the engine was running well. Last week I even spotted fine bubbles at high throttle. I suspect the bubbles were always there and I just hadn't noticed. I generally don't try to find out what's wrong with a good running engine.
A couple of days ago I put in a new needle "O" ring anyway because I was sure there was a leak. It still had the bubbles and it still ran really good. After the flying session I started to drain the remaining fuel from the tank and noticed I was getting almost solid foam. I had hooked the fuel pump to the line between the carburetor and the needle instead of the needle and the tank. As a result the pump was sucking the fuel out of the tank through the needle valve. Then I noticed that the fuel coming out of the tank had no bubbles. It was solid fuel. Conclusion, big leak in the needle valve requiring fixing, broke or not. Latter that day I put a clamp on the line from the needle valve to the tank and drew a vacuum on the line to the carb. It held the vacuum, NO LEAK. I filled the tank with fuel and then began sucking it out through the needle valve with the pump. Sure enough, foam again, but still the line to the tank was clear fuel with no bubbles. I left the pump run and clamped off the line to the tank. All flow of fuel, bubbles, foam, air or anything else stopped coming out of the needle valve. Again NO LEAK.
Now I'm really beginning to believe that what I and maybe many others have been seeing in the OS remote needles and YS 4 strokes is alcohol vapor bubbles and not air bubbles caused by leaks. I not sure it makes a difference if it's alcohol and not air. Uneven fuel flow is still uneven fuel flow.
Any comments? Any fluid dynamics persons out there that can give some insight? Fluid properties folks? I'm an aerodynamics major but that's just a small segment of fluid dynamics and doesn't help me here a bit.
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I never met an engine I didn't like.
Of the things I've lost I miss my mind the most.
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YS91 - Will not idle / bubbles in the fuel line - 
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YS91 - Will not idle / bubbles in the fuel line - 
RE: YS91 - Will not idle / bubbles in the fuel line - 