Mercury Carbs

A manual is very helpful. My Seloc manual gives step by step disassembly and reassembly instructions. It also has parts diagrams.

Last week I pulled the 3 carbs off a 1970 135 hp Merc that I've owned for 2 or 3 years. It had been sitting for about 6 years before I acquired it. I was told it had ran before but I bought it from a used car salesman so who knows. I only planned on cleaning the carbs and replacing the gaskets so I only did a partial disassembly. I removed the idle screws, the floats, and float needles. I didn't want to risk screwing anything else up so I didn't try to remove any of the jets, plugs, or seats. These parts are all brass and are removed with a slotted screwdriver so they are easy to damage. I cleaned everything with aerosol carb cleaner using the plastic straw to blast out all the jets and passages. I reassembled using new gaskets.

I did have to go back and open one carb up and replace the float needle and seat because it was leaking fuel when primed. The float needle seat has a hex head and it came right out with a 6 point socket.

It must have worked because I hooked the fuel line up with new fuel in a tank, primed the bulb, cranked the starter, and it fired right up. I was surprised as well as quite happy. Remember this motor had been sitting for at least 8 years. I let it run at idle for about an hour and it sounded pretty good.


One tip when removing the idle mixture screw. Before removal mark the slot position and then count the turns clockwise until the needle seats. DO NOT overtighten. Remove and clean. When reinstalling seat the needle gently and then back off the number of turns you counted earlier.

 

I forgot to mention I used both Mercury and Sierra parts. The Mercury gasket kits were only about 8 or 10 dollars for each carb. Merc fuel pump kits with check valves were $6. The Mercury parts house was short one fuel pump kit so I stopped at the local NAPA to order one and they had one on the shelf. They also had the float needle and seat on the shelf when I needed it.

The owner is a friend of mine so I had to apologize for all the grief I've given him for not having truck parts in stock when needed. I never dreamed they'd have parts in stock for a 34 year old outboard.

 

If fuel is pouring out of the throat when you prime the bulb your float needle and seat should be replaced. If the fuel is leaking out of the bottom of the bowl you just need a new gasket on the screw.

Either of these repairs is easier than a complete rebuild, I wouldn't even fool with taking the idle mix screw out. Just disassemble the major parts, blast everything clean, and replace the gaskets when reassembling.