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Technical question

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Technical question

Post  Cox International on Wed Mar 29, 2017 11:52 am

We have received a question as per below (Purple Python = similar to Venom - Black Widow - Golden Bee, etc). We are thinking excessive vibration caused by imbalanced prop but does anyone have any other thoughts?

I wonder if you can help me with a technical problem. On the Purple Python when it is running it sucks fuel out of the filler pipes, it is acting on the same principle as a spray gun where the air going over the pipe sucks the fuel out, thus reducing run time. Have you come across this problem before and if so how can I cure it?
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Re: Technical question

Post  wha-tah-hey on Wed Mar 29, 2017 12:31 pm

Reed not sealing, acting as a pump?
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Re: Technical question

Post  fredvon4 on Wed Mar 29, 2017 1:15 pm

Tank to case or back plate seal, screws, or venturi tube seal leaking in air, and as noted by what- tah- hey...reed seat

Make sure stunt tank vents are vertical not horizontal low or high once all air leaks are solved

Out of the box all my Cox #55 specialty engines had good fits but I did need to face off each crank case and make sure each screw did not bottom out

In My case I always chase the case threads deeper with a bottoming tap and use Ken Cooks Que Tip cotton trick on the screw heads

I also find that the plastic back plates are very easy to distort if the 4 screws are torqued too tight

Another trick I just do for no reason... is a short piece of fuel tube with 45 cut facing into air-stream (my thinking is / was ram air into tank)



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Re: Technical question

Post  Surfer_kris on Wed Mar 29, 2017 2:24 pm

I have seen this before and in the 80-ties I made a brass backplate with two soldered copper tubes that would slide over the inlets on the Babe Bee. The tubes were bent forward for a ram-effect to keep the fuel in the tank. It didn't really help much and the conclusion was the the fuel only comes out if there is a leak to the crankcase volume. This can come into the tank around the backplate screws. So the key thing is to remove the ridges that comes from the treading for the backplate screws (use emery cloth on a piece of flat glass) and to use a tank gasket towards the crankcase. Once this is sorted out there should not be any fuel coming out of the vents anymore...

To be sure one can also block one of the tubes, as only one is needed to let air in.
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Re: Technical question

Post  andrew on Wed Mar 29, 2017 3:29 pm

fredvon4 wrote:

Out of the box all my Cox #55 specialty engines had good fits but I did need to face off each crank case screw and make sure each screw did not bottom out
andrew: Yup to this -- bottoming screws can be a hidden problem.

I also find that the plastic back plates are very easy to distort if the 4 screws are torqued too tight.
andrew: Yup, again.  The distortion may not show up immediately, but over time the backplate will dish inward.

Another trick I just do for no reason... is a short piece of fuel tube with 45 cut facing into air-stream (my thinking is / was ram air into tank).
andrew: And three Yups for fredvon4 -- this is a quick and dirty fix. If there are two filler tubes, block one off.

Lastly, an out of balance prop can cause fuel foaming and increase the amount of expelled fuel.
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Re: Technical question

Post  Cribbs74 on Wed Mar 29, 2017 8:47 pm

It's a siphon effect as the owner noted and it does happen during normal operation.

His concern is the run time, which is not because of the small amout of fuel being siphoned off. It's because high performance engines are not fuel efficient. The faster the engine spins the faster it sucks fuel.
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Re: Technical question

Post  wha-tah-hey on Wed Mar 29, 2017 9:01 pm

So a simple solution is a notched section of fuel line between the vents.
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Re: Technical question

Post  Marleysky on Wed Mar 29, 2017 9:50 pm

wha-tah-hey wrote:So a simple solution is a notched section of fuel line between the vents.

Unless your working a government job, sometimes the simple solution Is the best one!
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