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Mylar reed valver failure Empty Mylar reed valver failure

Post  Iceberg on Thu Jun 20, 2019 7:37 pm

I have one .049 silver bee that was running well then started suddenly running short time. Found the fuel line had risen in the tank that was the reason for short run. Then is started coughing out after 30 seconds - 1 minute.

After several tear downs put new venturi gaskets etc etc still did the same. Sanded the back of the crankcase with 1000 grit. put double gasket there. Problem persisted.

Took apart again.....

Sucked on the reed valve and discovered it wasn't sealing properly on compression stroke. I have changed it now to another brand new one. Has anyone else seen this happen? Is that the way a bad reed valve behaves? Just coughs and loses fuel suction?

Thanks
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Post  Ken Cook on Thu Jun 20, 2019 8:28 pm

The Mylar version happens to be my favorite version. It offers good performance but it can offer problems as you mentioned. The center has a tendency to suck down into the venturi hole distorting it. Sometimes all that's needed is to flip the reed over and try the suck and blow test. I use large syringes to do my test and at no time should any air pass when withdrawing the syringe. I've even had brand new ones fail this test regardless of orientation and I discard them.
One reason I prefer the Mylar is that they have a tendency to squeak like a squeaky dog toy when the prop is flipped. This lets me know the reed isn't stuck and is functioning. I also wrap the fuel pickup line on the nipple with copper wire and twist it up until it's snug. This insures that it's not going to move or come off. I have never found the Cox offered tubing to work long term.
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Post  Iceberg on Thu Jun 20, 2019 8:42 pm

Hi Ken
Thanks for that. The new reed I put back is actually a cross copper one now and it seals beautifully. It actually does a slight "squeak" as you mention too.

I was going to include a picture but didn't take one. I often will use very thin copper wire also and tie the bottom part of the pick up tube perfectly in the 8 o'clock position. I tie it around the one screw for attaching the tank that is at the same spot. I do this because I can't get hose here in Korea and often have to resort to using medical tubing for intravenous and they tend to "pop up" horizontal. But by tying them to the tank screw very lightly they seem to work very well. I have a few Golden Bees that will run 65 runs around the circle using that system.

I also have discovered recently another trick that I've very impressed with. I use diskette film to cut gaskets for the venturi. Sometimes on its own sometimes sandwich a real venturi gasket between two pieces. It really works well and the fuel does not dissolve the diskette.

I used to make reed valves out of diskette and actually they ran excellent to almost 20,000 rpm before they floated.

In any case Ken

Thank you!
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