Troubleshooting a Cox Black Widow engine

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.

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

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?

I would have never thought a piston would cause this!

Thanks for that bit!

Live and learn, through other people's experience.

FJ

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.

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 !!