Mid 80's 70 HP Merc Running On 2 Cylinders

[jcleveland78]

I have a mid 80's (not sure of exact year) 70 horse Mercury outboard that recently started giving me some problems. When the engine is cold it is hard to start. Once it does start, it has to warm up for several minutes before it runs properly. If I give it full throttle soon after starting, it will only run on 2 cylinders and not even get up on plane. I hear the 3rd cylinder firing every once in a while and it will jerk forward, but immediately resume on only 2 cylinders. After I let it sit and idle for five or so minutes it is usually alright. If I turn it off for a while (even an hour) and re-start it runs fine. I have checked and all three cylinders are getting a spark. I have also replaced the plugs, but the problem persists. I have a suspicion that my fuel is too rich and that the third (bottom) cylinder is flooding. I don't know how to adjust the fuel to make it leaner. If anyone can help or offer suggestions I would certainly appreciate it. I guess my next step would be to replace my plug wires and possibly the coil.

 

[schaiss]

Re: Mid 80's 70 HP Merc Running On 2 Cylinders

Have you done a compression check recently on that engine? where you say there is a good spark on all cylinders would tend me to rule out spending any money on electrical parts. A slightly lower compression on the third cylinder could indicate worn rings, which would tend to expand and seal better as the metal heats up. To confirm this, with the engine cold try squirting a little bit of 2-stroke oil directly into the suspect cylinder and the try starting. The oil should seal slightly worn rings for an easier start.

 

[Merc805]

Re: Mid 80's 70 HP Merc Running On 2 Cylinders

Is it one of those 4 carb (2+2) motors ? Or just the 2 carb 4 cyl standard 2 stroke.<br />What is your S/N ? We can find out what you have.<br />Also, be very careful running it too much with this type of problem, as it could be caused by a "Lean condition" which can burn your pistons.