Mercury Outboard 15hp carburetor problem

Bill & Judy Rouse <yahoogroups@...>


Thanks for your email. It was difficult because of a series of connected and disconnected problems. The following were the issues causing and/or contributing to the loss of power:
  1. Fuel line pump-bulb had a small hole (air leak)
  2. Fuel lines (2) inside the engine cover were disintegrating from the inside (not obvious on the outside) this caused very small pieces of rubber to get inside the carburetor and it also caused the fuel line to eventually crimp under vacuum.
  3. Fuel vent on the fuel tank...the o ring around the vent plug had shifted and was actually blocking the two holes in the plug, thus stopping all venting.
Number 2 above was the tough one to find and fix. The tiny bits of rubber got lodged in several places: 
  1. The diaphragm fuel pump valves
  2. The float fuel valve
  3. And the very tiny jet that goes from the bottom of the fuel bowl to the venturi.
In all, it took five or six carburetor removals and overhauls to find all of the problems. Finding 7mm fuel hose in Montenegro was a challenge. A taxi and a search of one small town, then another larger town produced 8mm fuel hose for 2 euro, plus 45 euro for the taxi. Fortunately I had saved the fuel line pump bulb from 5 years ago when I changed out the entire fuel tank hose, so I simply am using the old one because finding a new one here is impossible.

The fuel filter did not help when the main contributor to the debris was after the fuel filter. When I get a chance, I will add a disposable in-line filter immediately before the carburetor.

The entire 1 week process was very frustrating and had me considering replacing this 11 year old motor. In the mean time, I did order the reed valve assembly, another carburetor kit, and a few other things which are waiting for me in Houston. I had planned to have them shipped to me, but will wait for a while. 

I could not have repaired and cleaned the carburetor if I did not have a compressed air line that I connected to my dive tank. This line is a new addition for me. I bought a 50' lightweight hose from a dive shop that will screw into one of the 3 places on my "first-stage" regulator. At the other end of the hose I can connect either the "second stage," or octopus that I removed to connect this new line, or a air gun. In this case I connected the air gun. I also bought it for quick under-the-boat diving where I can leave the tank on-deck.

I had 4 or 5 years of 2 cycle engine experience when I was a teenager. I raced a go-cart which topped-in at 80mph, powered by two chain saw engines, lots of modifications, and a mixture of gas/nitro. For every hour of racing there was at least 12 hours of repairs/work. In that environment, I would get less than 1 year on the reed valves...but, of course, when they were working the engine was at 12,000 - 15,000 rpm.

I hope that this helps someone with similar issues.

BeBe 387

On Mon, Jun 30, 2014 at 12:22 AM, gmshea@... [amelyachtowners] <amelyachtowners@...> wrote:


       Did you fix the outboard problem? The fuel tank vent would have been my first choice, as someone pointed out. Was that it? Considering all of the other stuff you've done, I'd suspect the fuel pump. I'm not conversant with the Mercury pump specifically but they are normally a diaphragm type driven by the crankshaft pressure cycles. If the diaphragm stretches or develops a leak, you will get your symptoms. Pressurizing the whole fuel system with the primer bulb will get you going but if the pump doesn't do it's job, the motor will die after a few minutes. Try getting it going and see if it stays running if you continuously squeeze the primer bulb. 
       I don't think the reed valves are a suspect. They are usually very long lived unless you damage them during a tear down. 

Greg Shea
Sharki 133