Mercury Classic 50 starts but then dies....

mattsk7

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Oct 6, 2013
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I noticed that the motor was sputtering a bit at high RPM while in the water. I rebuilt the carbs last week hoping that it would fix the issue. After starting it takes about 10-15 seconds for water to circulate through the engine and then it will die within 30 seconds to a minute. It is idling great and sounds like everything is fine and then a rough, slower idle and then kaput! I have been trying to tweak the idle screw to see if that is part of the issue with no success. I just cleaned the plugs last week and took a look at them again yesterday and they are filthy (picture attached). Any help with this would be greatly appreciated!!
IMAG0248.jpg
 

hall832

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Re: Mercury Classic 50 starts but then dies....

that looks like water, are they wet?
 

mattsk7

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Re: Mercury Classic 50 starts but then dies....

that looks like water, are they wet?
They do not appear to be wet. It looks/feels like oil and fuel. I'm not sure if you can tell but it is a blackish sludge. I thought this was odd considering the oil is blue.
Thanks for the response!
 

hall832

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Re: Mercury Classic 50 starts but then dies....

I suspect you still have issue with carbs. And the black sludge is normal, just maybe decarb once your issue is fixed.
 

mattsk7

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Re: Mercury Classic 50 starts but then dies....

I figured as much. I'm just not quite sure what the solution is. It's beginning to drive me a little crazy...
 

gpfishingdude

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May 1, 2012
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Re: Mercury Classic 50 starts but then dies....

I changed the stator and trigger on mine first because the wires from them were corroded and probably shorting out. Then I checked the compression and the spark at each cylinder and figured out what was bad by switching the coils on the good cylinder for the bad one to make sure it was a coil and not a switch box causing the misfire. If the plug is wet it probably is because that cylinder is misfiring but a spark test will tell you for sure.
 
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hall832

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Re: Mercury Classic 50 starts but then dies....

did all the plugs look the same
 

mattsk7

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Re: Mercury Classic 50 starts but then dies....

All the plugs look the same.
I did a compression test and everything checks out. I'm still thinking that it is fuel related, probably something with the carbs. It runs great for a bit so I don't think it is related to the spark. I have taken them apart a couple of times and everything looks good. Any ideas would be appreciated!
Thanks for the responses.
 

CharlieB

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Apr 10, 2007
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5,617
Re: Mercury Classic 50 starts but then dies....

Service your water pump, it shouldn't take but 3 or 4 seconds to fill the engine with water. Once you get this running great you sure do not want to burn it up.

Disconnect the fuel hose at the carbs, point it into a suitable container and crank the motor with the lanyard pulled. You just want to see the fuel flow a nice fat pulsed stream, any less and the motor will run low on fuel, possibly running the top carb dry and burning the piston.

Second test, replace that hose with a clear hose, reconnect it, and start the motor, on the hose or in the lake. Watch the clear hose for air bubbles passing thru indicating air sucking into the system. Again, this will allow carbs to run low on fuel, the motor either dies (hopefully) or runs lean, burning the pistons.

REMEMBER, remove that clear hose once finished testing, clear hose is NOT rated nor approved for fuel use. Only as a short term testing tool.
 

quicktach

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Aug 24, 2012
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Re: Mercury Classic 50 starts but then dies....

The picture makes me think that's burnt oil on the plug. I'd focus on a carb problem for now. Flooding ?
 
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