87 70hp Merc Carb or Fuel???

[sbb1933]

I have a 87 70 hp mercury 3 cyl that idles kinda rough but when I give it full trottle it bouges down.I can move the trottle forward a little bit at a time and it will start to clearing out and getg on plane.I bought it about 4 yrs ago and I haven't had any work done to it.When I adjust the carb screws I can turn the middle carb screw all the way in without any noticeable changes in the idle of the motor.I wonder if it needs carbs cleaned or maybe fuel pump may be not supplying enough fuel.

 

[Chris1956]

Re: 87 70hp Merc Carb or Fuel???

I sounds like a fuel problem. Check the fuel pump diagram to see if it leaks. It is likely OK, but you want to eliminate that. If the idle mixture screw had no effect on the idle, either the carb is clogged or the cylinder is not firing.

Do you get top speed? If so, it is likely the idle mixture on the mid carb is so whacked out, or the idle passages are clogged.

 

[sbb1933]

Re: 87 70hp Merc Carb or Fuel???

It will get to top speed gradually and you can hear it cutting in and out as it picks up speed. Thanks

 

[Grant Brown]

Re: 87 70hp Merc Carb or Fuel???

I have the exact same motor - 1987 Merc 70 3 cyl.
It did a very similar thing for the last 2 years - I had the carbs rebuilt twice, changed plugs, etc. etc.
It would fire and idle fine, as soon as I got on the gas it would bog and take forever to get up on plane. Top speed was only 25 and it does an easy 35 when it runs good. I'd stop and drop crab traps or fish and it would heat soak for awhile. When I'd fire it up again it would run like a scalded cat for the rest of the day - amazing hole shot etc.
Turned out to be a switch box. I bought a no name from epay for about $75 shipped, installed it in under an hour and problem solved.
Runs like a different motor now.