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Author Topic: Cox Fuel tanks  (Read 609 times)

Offline Ronald Eshleman

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Cox Fuel tanks
« on: October 11, 2017, 06:55:37 PM »
I have been working on several Cox 049's.  Keeping them running once in the air is a problem.  We are using new Cox fuel from Brodak.  I replaced the glow plug. Tightened screws and cylinder. Cleaned tank. Moved the inside tank tube to the outside side of the needle valve.  We even added an additional compression ring in 1 motor.  Still will not run a full tank of fuel... Also, any ideas on how to work with a 1 vent tank?  Never saw this? How do I know when the tank is full?  Thanks

Online Gary Dowler

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Re: Cox Fuel tanks
« Reply #1 on: October 11, 2017, 07:38:02 PM »
Never seen a tank with only one vent. They all had two. Two styles were out there, one with a fill and overflow on the tank itself, the other had two vents on opposite sides of the needle valve.  These were difficult to fill as the fuel vents were very small and 1/8" fuel hose just won't fit them. Best tactic is to get a syringe with a 18 gage needle from a feed store and use that.
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Offline Dan McEntee

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Re: Cox Fuel tanks
« Reply #2 on: October 11, 2017, 07:50:55 PM »
    You may be seeing the fill tube, but not seeing the little pin hole that acted as an over flow on the other side of the needle valve, One simple thing to check when youhave the tank off the engine, is to replace the o-ring that seals the venturi tube to the backing plate, These shrink with age and when they don't seal well, you don't get a good fuel/air mixture. Also make sure the needle valve passages are all clean and clear of varnish. The reed needs to be flat and sealing properly, and still flutter freely. The gasket between the tank and the crank case is important also. Make sure the needle valve isn't bent. A short length of fuel line helps seal the threads on the needles valve also. All of these parts can be purchased on line or good used parts from donor engines used. I've made new o-rings by slicing them off a length of the black fuel line that some of the RTF cox models used. Once you tinker on them a bit and figure out the finer points, they are easy to work on and get to run well. There is a 1/2A section of this forum and the Cox Engine Forum also where there is untold wealth of knowledge on these little jewels.
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Offline Phil Krankowski

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Re: Cox Fuel tanks
« Reply #3 on: October 12, 2017, 06:53:36 AM »
I'll put this here to start.

There are many styles of back plates over the years, and several different fuel fill arrangements.

The one-nipple back plate has a "pinhole" adjacent to the fill nipple.  It can be blocked easily by the rubber hose for filling or debris.  Both cases will make filling the thank difficult.  The first condition, blocked with the rubber hose, can be solved easily by having a less than square end on the fuel fill hose.  This will allow a tight fit over the nipple, and prevent the hose from seating all the way down so it will not block the vent.  Bubbles and fuel will come out around the fill when the tank is full.

If you suspect debris then carefully use a pin to carefully clean the hole.  This is better done with the engine taken apart.  Magnification may be needed. 

You can see light passing through the vent, adjacent to the fill nipple in these product images.
better top view but the vent is less visible.
There are metal backs with this fill/vent arrangement.

The O-ring on the venturi is a bit difficult.  They can be made out of medium silicone fuel line by putting a short section onto a skewer and slicing rings off with a razor blade.
although buying new is an option too.

The seal around the back plate to the fuel tank bell is a little more difficult.  There are old recommendations of using floss or cord to provide a filling in the tank groove so the seal is better.  This does work but I gave up after a dozen to so tries and simply fill the groove in the aluminum tank with anaerobic sealant from the local car parts store.  I then clean the excess off and the inside of the tank.  When assembled this will provide an excellent seal against air.  I also use some of the same sealant on the venturi gasket and crankcase gasket. 

The last major problem is air leaks around the screw heads in the back plate.  A little more sealant _under_ the screw heads resolves this quite well. 

Finally if you suspect that fuel draw is still an issue, and you have cleaned or replaced the fuel draw spring (functions via capillary action) and fuel line  then look up the
Gibeault Mouse Race Program
and replace the pickup with a metal line as indicated.

Hope this helps!