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Troubleshooting a Cox Black Widow engine

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Troubleshooting a Cox Black Widow engine

Post  Cox International on Tue May 23, 2017 3:06 pm

Issue:
Engine runs for 30 seconds then surges, sputters and dies. Large needle valve adjustments may extend the run by another 10-20 seconds.

This has been done / tested:
New glow head and gasket
New venturi gasket
New crankcase gasket
New fuel tank pickup line and spring

Good compression
Glow head burns bright
Needle valve seals
Fuel tank pickup line is located at rear bottom of tank
Fuel is good (25% Nitro)

Finally found something else that needed attention but wanted to get your input before I continue (just to see if what I found would have contributed to this)

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Re: Troubleshooting a Cox Black Widow engine

Post  fit90 on Tue May 23, 2017 3:20 pm

You might want to try cleaning and reseating the reed and blowing out all the fuel passages. It sounds like it could be having trouble getting enough fuel from the tank into the engine.
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Re: Troubleshooting a Cox Black Widow engine

Post  Ken Cook on Tue May 23, 2017 3:21 pm

Sounds like the reed. I always air test before re assembly. The tank style that utilizes the circlip makes this very easy. The newer style doesn't. Use a large syringe hooked to the back of the venturi and try and draw the plunger back. You shouldn't be able to AT ALL. Take apart, flip the reed over and try again. Don't think that just because a reed is brand new it seals, many times they're concave and which is why I suggested flipping it over. If equipped with a true stunt tank, make certain the vent pipes are totally clear by pushing a wire through them.
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Re: Troubleshooting a Cox Black Widow engine

Post  Forgetful John on Tue May 23, 2017 3:34 pm

Perhaps there is some muck in the backplate? Remove needle valve and spring. Alternate suction and pressure applied to needle area with a syringe of fuel. Watch for fuel to come out into small hole in venturi area, with the fuel line removed and plugged with your finger. I once found a broken off tip of a needle valve in there.

Also, check the back of the crankcase for flatness on a sheet of glass. This can be trued up with some very fine sandpaper. And since you are right there, see if the four back plate screws are going all the way into the crankcase. If the screw(s) are just the slightest bit too long, there is swarf in the screw holes or the back plate has been deformed from excessive tightening, even on the metal ones, the screws will feel tight but not be exerting adequate clamping force.

FJ
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Re: Troubleshooting a Cox Black Widow engine

Post  Cox International on Tue May 23, 2017 3:56 pm

Forgot to mention that the reed was changed and the reed cavity cleaned.

Engine is brand new and the backplate seals well. Also, the filler tubes are clear.

It's none of the above but what I finally found was a piston with excessive ball-socket play; and I mean excessive. After replacing the piston, the engine ran fine.

Could this have caused a problem as mentioned in the OP or did I simply luck out with a different piston?
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Re: Troubleshooting a Cox Black Widow engine

Post  Forgetful John on Tue May 23, 2017 6:33 pm

I would have never thought a piston would cause this!

Thanks for that bit!

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FJ
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Re: Troubleshooting a Cox Black Widow engine

Post  Cox International on Tue May 23, 2017 7:50 pm

Forgetful John wrote:I would have never thought a piston would cause this!

Thanks for that bit!  

Live and learn, through other people's experience.  

FJ

We are unsure if a piston could cause this; hence this post.

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Re: Troubleshooting a Cox Black Widow engine

Post  getback on Tue May 23, 2017 8:53 pm

Brand New !?!!!? That was my first wonder , WITH THAT MUCH PLAY IN THE SOCKET sorry but , what John was saying , the hole at the back plate to venture is Very small and don't take but a smidgen of Scarf from drilling to make a big difference>> and you had a lack of fuel for sure saying you could riching the blend and get more run time . I think you just pulled it through !! Babe Bee .049
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