bee overflow question

Of a stunt tank is combined with a backplate that also has filler nipples, the fuel will be easier sucked out by the depression in propeller air flying over the nipple. Just my guess, I am not sure what backplate you are using.

Seems also to disappear once the Bee's RPM is up. I wonder if at lower RPM's, if the venturi gasket is missing or compromised where it isn't fully seating, there may be a touch of pulsating from the reed assembly causing bubbling at the tank back, slighly pressurizing the tank, pushing fuel out the overflow.

Again, just a guess, but I think if you carefully disassemble the engine and re-examine everything, then will find the culprit.

I would go with checking the gasket and venture O-ring , just replace and try that .. Although they do ten to leak with air burst but usually not that bad .

Just guessing, but it could be that there is a resonance causing the fuel to be agitated in the tank. A test would be to try another propeller and/or a different test stand. Or clamp a weight to the test stand or its mount.

The backplate doesn't have its own vents, does it?

thanks everyone for great ideas and pointers

the backplate has no filler holes, so that isnt it, but I am liking the idea of checking the venturi gasket! and will do so tonight

thanks

one more cox back to life!

The only way positive pressure would reach the tank would be through case pressure. In order for that to happen, the reed isn't effectively sealing on the piston downstroke. In turn, it's now pressurizing the venturi which in turn would bleed out through the venturi screen but I can see it also passing through o-ring and into the tank. This could also happen if a crack formed on the tank. Closing the needle down and pressurizing the tank with a syringe will tell you immediately if the o-ring is sealing. It will also reveal other leaks such as tank to tank back, needle valve threads, and the 4 screws. Doing a suck blow test on the tank will tell you if the reed is properly seating. Attaching a syringe to the back of the tank with a piece of tubing, YOU SHOULD NOT be able to pull the syringe plunger back at all.

thank you Ken, I will be swapping the o-ring and trying the syringe method in a few minutes. thank you for good advice

I'm not certain as to your reed style, clip vs cap. Make certain the clip is installed correctly with it's 90 deg return leg facing away from the reed and not squashing it. The reed needs to fully rotate under the clip. Even a new reed might not successfully seal, you might need to turn it over and try again.