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  1. #1

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    Old Cox Black Widow Engine Not Running

    Hey guys, I'm new to this forum. Ihave recently re-entered the RC boating world after about 20yrs. Ihave an old Dumas airboat with a Cox Black Widow engine in it and I'm tring to get it to run. The engine will start but I can't get it to run for more than 20secods. The engine was only mildly used and the piston is free and looks to be in good shape. I also replaced the glow plug with a new one. I think the problem is with the fueling, but I should mention that I'm at an elevation of 7000 feet so I'm not sure what the best setting should be for the needle valve. Regarding fueling the tank, the manual says to fuel from the bottom port and then once the tank is full the excess fuel will come out of the top port. When I attempt to fill the tank, the fuel immediately starts to flow out of the top port. I should add that I was into the hobby as ayoung teen and my recently deceased father knew all the tricks when running these types ofngines. He even used to start them manually using the flyweel and a piece of string. I was too young to pick up onallthe nuances.In any case, does anyone have any suggestions? Thanks

  2. #2
    crispyspa's Avatar
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    RE: Old Cox Black Widow Engine Not Running

    I wonder if the fuel tank is on upside down.

    If the engine is getting super hot, it's too lean. Start with the needle out 3 turns from closed.


    After all is said and done, more is said than done.

  3. #3

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    RE: Old Cox Black Widow Engine Not Running

    I'd try new fuel lines, new fuel, and make sure the tank is not leaking, and run her richer as Chrispyspa suggests. Your elevation will mean that you need to dial in richer than settings at sea level because the air is thinner. But if this motor has sat for a period of time, the carb could be gummed up and not running right, the motor could have air leaks, and so on and so on....

    How is the throttle response?  is it 20 seconds at full throttle?  or idle?  Does she start straight away after a stall?  or do you have to re-prime the fuel ?
    How old is your fuel ?
    There are heaps of potential problems, but don't give up! 
    You can also post in the Glow Engines forum


  4. #4

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    RE: Old Cox Black Widow Engine Not Running

    Thanks for the response guys.  I haven't gotten it running long enough to play around with the throttle response, so that's 20 seconds after initial start up with the needle valve at 3 turns.  Sometimes it does turn over after starting without re-priming, but it only runs for a good second or two.  The fuel is brand new.  Will have to play around some more.

  5. #5
    MajorTomski's Avatar
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    RE: Old Cox Black Widow Engine Not Running

    The black widdow has a tank vented for inverted flight. BOTH of the fill tubes go to the other side of the tank. So when you are filling by the LOWER tube you're getting just enough fuel in the bottom of the tank to vent out the top.

    Fill it from the TOP fill tube until fuel flows out the bottom tube. That should fix it.
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  6. #6
    crispyspa's Avatar
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    RE: Old Cox Black Widow Engine Not Running

    Your engine has a carb on it? Most of these little cox engines just run wide open until the fuel runs out.
    After all is said and done, more is said than done.

  7. #7
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    RE: Old Cox Black Widow Engine Not Running

    Here is a link to a test on the Cox .049 black Widow Engine from 1974.

    http://www.mh-aerotools.de/airfoils/...ugust_1974.pdf
    Kirkwood RC , NFMABA , MABA
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  8. #8

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    RE: Old Cox Black Widow Engine Not Running



    Thanks for all the info. Haven't had a chance to try again, but will give it a shot this weekend.


  9. #9

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    RE: Old Cox Black Widow Engine Not Running

    Good news.  With the suggestions from above, I was able to get the little engine running.  Problem seems to have been with the needle valve not being open quite enough and the tank not being propely filled.  I know I adjusted the needle valve to compensate for the higher altitude, but I think I overadjusted it.  Oh well, live and learn.  Thanks again for the help. 

  10. #10

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    RE: Old Cox Black Widow Engine Not Running

    Good to hear ! 


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