87 Mercury 25hp will start, run rough for a few seconds, then die

ccruiser91

Cadet
Joined
Dec 2, 2012
Messages
7
Good afternoon;

I have an '87 Mercury 25hp, sn 0b172098, model 1025557. I received the boat and this motor as a gift last December. After cleaning out the gas tank, filling it with fresh fuel and the correct amount of Sta-Bil, the motor started on the third pull. It ran roughly, but it ran. I figured it needed new spark plugs, which I replaced about 6 weeks ago.

On our first trip about two weeks ago, the thing wouldn't start at all. Finally, after an hour, it started, ran just as roughly as before and after about 5 seconds, died. It did this 15 times or so. Next thing you know, it started up, ran, albeit roughly, and I decided to head out (knowing the electric trolling motor was good to go). Then, for about 20 seconds, it was as smooth as silk! My two youngest sons cheered, then it died, just like before, and it went back to running roughly for about 5 seconds, then dying.

A couple of days later, I checked for spark and both plugs are showing good, blue sparks. I checked the fuel filter and it's good. I know the fuel's good. The bulb stays firm, so I'm guessing the carb and/or the fuel pump? Here's the next part of the problem though: money's a little tight and I can't afford to get to many non-returnable parts in the attempt to get the motor running and was hoping you guys might be able to help narrow down the possible culprits.

Thank you so much for your assistance!
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: 87 Mercury 25hp will start, run rough for a few seconds, then die

You didn't mention adding oil. Does your motor have an oil tank, if not 2 stroke oil must be added to the fuel at a 50:1 ratio to provide lubrication of all moving parts.

Lack of oil will destroy the roller bearings, crankshaft, rods, and pistons in a hurry.

The small idle passages can clog up with oil residue when the fuel evaporates during longer periods of non-use. It may need the carbs cleaned to restore proper fuel flow.
 

ccruiser91

Cadet
Joined
Dec 2, 2012
Messages
7
Re: 87 Mercury 25hp will start, run rough for a few seconds, then die

Thank you for responding! Indeed, I should have mentioned that I added the Quicksilver brand of 50:1 2-cycle oil to the fuel tank along with the Sta-Bil before I added the gas. So maybe just a carb cleaning is in order?
 
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