1987 mercury 20 hp idle and top end problem

[elliottm82]

Hi, I have a 1987 mercury 20hp (0B223800) and I am having trouble getting it to idle without dying out and It will only run at 2/3 of it's original speed. the throttle linkage fully advances fine but The rod that connects the throttle linkage to the timing adjuster is gone. Ran motor full throttle today and manually advanced timing with my finger under the flywheel but did not make any changes in motor. In other words, still did not have full throttle. Could this have something to do with it?? Someone mentioned a switchbox, could this be the problem?? Or does it sound like a carb problem or some other problem??? It has relatively new plugs and carb was reciently rebuilt. The motor runs good above idle and below top end. Should run 27 mph and it is only doing about 15 mph tops. Any help would be great, thanks.

[Laddies]

You need to start by checking to see if you have spark on both cyls. as it sounds like you may only running on one.

[elliottm82]

Ok, I checked to see if both cylinders are firing, and the bottom is not. I am getting a spark to the plug but it must be a weak one. I swapped the plugs around with the same result. Weak Spark maybe?????

[Yabba=Dabba-Doo]

I seem to have a similar problem with my Mercury Thunderbolt 20 HP engine. It idles with high revs and loses top end and dies. Did you sort out the problem with the bottom spark plug not sparking? and was this the cause of it the whole problem?