Re: Cox 020 PeeWee rebuild questions

balogh Mon Nov 11, 2024 1:36 pm

Dave, before applying the CA you may want to do an underwater leakage test in a glass bowl filled with water and the tank and backplate submerged with the crankcase and cylinder possibly staying above water. If you crank the piston halfway up the cylinder, then the crankcase is closed by the piston. Pressurizing the tank with air pushed into it with the syringe may help you detect where the air leaks i.e. between the insert and backplate, or along the NV threads? A hairdryer will help you dry the engine afterwards.

A leakage between the insert and the backplate is not critical..remember the fuel filling nipple is right next to it through which the tank interior communicates with the outside, anyway...the NV thread.leak needs to be doctored though..

LooseSpinner99 Today at 2:59 pm

Hi All,

I performed the underwater leak check proposed by balogh. I could not find a clear (enough) bowl or container to perform it in so I used a shallow plastic bowl with about a 2 inch inspection mirror in the bottom. See attached photos. It worked great, except I got leaks I'll cover after the pictures.

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I filled the bowl up to just below the cylinder with the engine sitting on the backplate. When I lift up the motor and tilt it slightly, I can see the backplate perfectly in the mirror.

Now the bad news. I saw that I was still getting slow leakage out the air/screen inlet. Very slow: a bubble would form, long pause, then another bubble, and so on. After several tries I could still not specifically locate where the leakage around the NV was coming from insert/backplate or insert/NV threads. I think I'm going to try to use some CA glue at the insert to backplate and re-leak check. Balogh, I'm not sure I understand how you mean to use silicone fuel tubing to help seal the NV threads if that is the problem. Can you please post a picture? Thanks!

The good news is, this is the second time I've used the Teflon tape to seal the backplate screw heads and they did not leak. I'll post pictures of what the disassembly condition is of the Teflon when I disassemble the engine. That means I didn't just get lucky the first time it worked. When I do it a third time and get no leakage, I'll be pretty confident in the method. Also, no leakage was noted at the fuel tank/backplate joint or the fuel tank/crankcase joint.

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Putting the Teflon tape, rolled into a ball, down into the two top crankcase screw holes seemed to work very well. I started with lengths of Teflon tape about 15-20mm. The balls were too big. Finally, once I got down to trying tape lengths of 5-7mm, the rolled-up balls fit just slightly snug into the screw holes. I'll show pics of what the breakout holes in the crankcase look like as well once I disassemble the engine.

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To address the leakage observed from the air inlet, that has to be coming from the venturi rubber "washer" seal I plan to try and fill the pore in the backplate mating surface for this seal. I'm also going to install a new seal. This seal has been through several assembly/disassembles and maybe not its best now. Also below is an older picture (last week) of the seal in position on the fuel tank. You can see that it does not fit completely into the seal groove area. It appears "loose" at both the inner diameter and outer diameter of the seal. I suspect that pressurizing the fuel tank may "push" the seal out of center and potentially uncover the casting pore in the backplate. I have a 10 pk of new seals coming. So, I'll work on the backplate pore first. Once I get the new seals,
I'll reassemble and try again. I will re-run the Sharpie mark matting contact witness mark test again, making sure it looks good before re-assembly.

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I hope I'm not boring anybody to death with all this detail!
Thanks,
Dave

balogh Today at 4:26 pm

Dave, did you turn the needle valve into fully closed position before you pressurized the tank? Air from the pressurized tank may leak through the fuel line into the venturi if the NV is not closed, so the bubbles appearing in the air intake may not be necessarily escaping from the tank around the O ring..

Below is a photo of how I would seal the NV threads with a silicone tube:

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You can remove the NV spring for good, but cut the tube end perpendicular to its axis so that it sits with its end flat on the face of the backplate top and seal well. Select a tube whose internal dia is not more than the outer dia of the NV so that the tube seals the threads too.
Cut the tube to the length where it will be snug when the NV is open to about 5 turns, so that when the engine runs at its peak at about 3-4 turns the tube will be slightly compressed and not only seal but act as a spring, too, preventing NV self-rotation due to vibrations.

LooseSpinner99 Today at 4:36 pm

Balogh, I haven’t disassembled the engine yet. I thought I closed the NV. I just checked and it feels closed. The original NV was slightly bent so this is a new NV. It doesn’t mean the NV isn’t leaking even if it’s closed and new!
Dave

balogh Today at 4:39 pm

When you disassemble the engine, pull a fuel line on the fuel nipple and pressurize it with a syringe with the NV closed, under water, and check if the valve is closing well or not. Bubbles should not appear at the air intake if the NV closes well.

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Cox 020 PeeWee rebuild questions