Cox Tee Dee .020 red Delrin intake sleeves

[Bill Schmidt]

The red Delrin front sleeve for the needle valve/ venturi assy. on these engines are quite old by now. Not only are they deteriorating due to hot fuels but are splitting behind and below the needle valve body. You cannot easily see this split as it is in the mold seam and hidden under the needle valve body. The 1st indication that you have an induction leakage is when you cannot draw fuel up to the needle valve when you choke the engine. The engine with only run out the prime over and over. If your are running pressure from an elastic type bladder you will not be as affected by the problem but the induction leakage will cause a fussy needle setting and possible erratic running. These little guys are more sensitive to irregularities than larger engines. If you put these engines away wet the 3ea. .017 distribution holes in the venturi will plug up with oxidized oil as well as the needle valve assy. Once again you will run out the prime over and over. Always carry a pointed .016 length of music wire in your flight box. After flying blow into the venturi using a length of fuel line with the fuel line disconnected and the needle valve open. The crank rotary valve must be closed. The excess residual fuel will exit the fuel inlet. You might suck as well as blow–there isn’t that much residual and this removes breath moisture.
The plastic fuel tank is terrible with its 2 hole mount and tapered fuel nib. I use a PEE WEE tank that has an aluminum disc. epoxied in the reed area and the nv and rear intake also filled shut. You must drill an .082 hole in the bottom of the tank and tap it 3-56. Bend a piece of 3/32″ K&S alum. tubing 90 degrees and cut the 2 ends to 1/4″+ with an X-acto knife on a table edge. Hand thread one end carefully shallow 3-56. Screw this end into the tank hole with Loc-Tite or epoxy. You now have a STABLE 4 point engine mount that you can offset thrust without the fuel leaking out the sloppy back end design of the original tank.
The replacement Delrin collars I obtained leaked air in the front and required auto gasket RTV on the threaded portion under the front screw on retention collar. They are not the tight fit of the OEM part. Check for air leaks by closing the fuel inlet and the crank rotary valve. Blow into the venturi using a length of fuel line while applying a rich soap solution with an acid brush. Leaks show up well using this method.
I have not looked into the situation on the Cox .049 -.051 engines but the above procedures will apply to them as well checking for induction leaks. Best, Bill

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