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Cox .049 behavioural oddness.

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Old 04-16-2013, 05:06 AM
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ZoomZoom-RCU
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Default Cox .049 behavioural oddness.

Methinks I have an airleak somewhere, but wanted some other opinions to help diagnose. Here's the setup. Cox .049 Surestart on the nose of a Wally world glider. Flies fine, has plenty of power but the engine is acting oddly. I have a carb throttle on it, which until now worked fine, as you all are aware this simply restricts the carb-air intake...you know the drill. So anyway last flight, I was flying then decided to throttle back for a landing because the engine started running erratically, pulsing up and down. I figured I was near the end of a tank so thats what I assumed it was. I go to throttle back, and nothing, engine stays in same pulsing cycle. Long story short, it stays up for some time doing this, until the tank really does run dry, and I land, assuming of course the throttle servo was not functioning ot disconnected. Upon inspection, the throttle was functioning just fine, opening and closing (and I did this many times during the flight with absolutley no response) and the plane was therefore running in this pulsing up and down cycle, and completely ignoring the fact that the air intake at the carb was being opened and closed. The only conclusion I can come up with is that shes got to be getting air from somewhere else to run like this no matter what the carb-intake is doing......thus....airleak-somewhere leak. What do you guys think? TIA!

ZZ.
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Old 04-16-2013, 07:13 AM
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Jim Thomerson
 
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Default RE: Cox .049 behavioural oddness.

First thing I would do is check the backplate screws to see if they are tight. Second would be the gasket to see if it was broken and leaking.
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Old 04-16-2013, 08:10 AM
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Default RE: Cox .049 behavioural oddness.

I agree with Jim, check the backplate screws. At the risk of ruffling some of the experts feathers around here I would suggest applying a very small amount of RTV silicone to screw threads. This will keep them from backing out. Don't go crazy just a little on the end of the screw will do. This method works well for the tanked engins also.

Also check the backplate for cracks especially if youve had a hard landing or two

Darren
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Old 04-16-2013, 09:02 AM
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Default RE: Cox .049 behavioural oddness.

Good advice. I will do so.

ZZ.
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Old 04-18-2013, 03:46 PM
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Default RE: Cox .049 behavioural oddness.

yes, check your backplate for tightness, also I've read somewhere that you should also check the back of the crankcase surface is true so you have no chance of leaks between the mating surfaces. One thing I have done to thecarburettor barrel carefully file down the length of the barrelto remove'end float'as I thought this couldbe an area that could cause erraticrunningeliminating any possibility of air leaking between barrel and control arm.
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Old 04-18-2013, 05:54 PM
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Default RE: Cox .049 behavioural oddness.

Some larger engines will continue to run when you close the throttle barrel.
The Webra .50 is famous for doing this.
I just adjusted my attitude about it and the problem went away. No tweaking or tinkering involved.
Keep the throttle wide open and enjoy the time you have on this Earth.
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