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Post  cstatman Wed Jul 26, 2023 10:39 pm

just got another one back to life

but in the video, you will see it runs okay, but is pushing fuel out the overflow

What am I missing? what would cause the tank to do this?

(it is running on fuel from the red tank, the clear fuel container behind it is not attached)

thank you

https://www.youtube.com/shorts/7tOEBh8nJGc
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Post  GallopingGhostler Wed Jul 26, 2023 11:54 pm

cstatman wrote:just got another one back to life but in the video, you will see it runs okay, but is pushing fuel out the overflow. What am I missing?  what would cause the tank to do this? (it is running on fuel from the red tank, the clear fuel container behind it is not attached) thank you

I'd run a fine wire through the tank overflow tube, just to be sure there isn't anything obstructing it like a bit of aluminum chafe or possibly a burr partially blocking an end, or even dirt or grease or sealant. This may be causing the tank to overfill above the top of of the vent tube.

Reason why I say this is because once it bleeds off after the first 5 or seconds of running, then it stops and the engine seems to run normal as expected.

It is just a conjecture.
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Post  balogh Thu Jul 27, 2023 12:00 am

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.


Last edited by balogh on Thu Jul 27, 2023 12:15 am; edited 1 time in total
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Post  GallopingGhostler Thu Jul 27, 2023 12:05 am

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.
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Post  getback Thu Jul 27, 2023 6:28 am

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 . Small Cox Logo
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Post  philosofriend Thu Jul 27, 2023 10:07 am

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?
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Post  cstatman Thu Jul 27, 2023 10:08 am

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!
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Post  Ken Cook Thu Jul 27, 2023 3:25 pm

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.
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Post  cstatman Sun Jul 30, 2023 12:48 pm

thank you Ken, I will be swapping the o-ring and trying the syringe method in a few minutes. thank you for good advice
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Post  Ken Cook Sun Jul 30, 2023 1:49 pm

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.
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