Early 70's 20hp outboard loses power at high RPMs

Hello all,

I've got an early 70's, sorry don't have the info handy, Mercury 200 20hp outboard that loses power at high RPMs. I replaced a bad ignition coil a few years back, and cleaned up and re-gapped the points last spring. The engine will start on the second pull, as it always has. It runs well until about 1/2-3/4 throttle, then it kind of sputters in and out. Like its firing one second, then not firing.

I've seen a few threads here that mention the high and low speed coils. Based on what I've read, I'm suspecting the high speed coil. Just looking to see if I'm on the right track with this. Also, can you tell me what the resistance should be for each coil?

Thanks!

Re: Early 70's 20hp outboard loses power at high RPMs

Sound more like a fuel problem than a ing, one. When was the last time that the carb/ fuel pump was cleaned or rebuilt?
Oldman570

Re: Early 70's 20hp outboard loses power at high RPMs

Hi Oldman570,

I cleaned the carb last summer. It was in very good shape. I didn't mess with the fuel pump, but I did change the fuel lines. I noticed one of the lines was pretty smashed and nearly kinked. After all this, it still did the same thing out on the lake. Good power and thrust until 1/2-3/4, but it won't get on plane at that speed.

The year before last it was running very well, but I do remember it losing power at hi speed a couple times. I remember getting off the throttle then back on it and the problem went away. That trick didn't work this year, which was why I was suspecting coil insulation breakdown. So if the fuel pump was going, and now it's really gone, it would lean out and lose power too?

Thanks for the insight.

Re: Early 70's 20hp outboard loses power at high RPMs

My memory was jogged when I looked up gasket/diaphragm sets for the fuel pump and I vaguely recall taking that apart as well. Those parts looked familiar. I'm going to say with 95% certainty that I did go through the fuel pump also.

I've also done the pump the primer bulb trick when the problem occurs, no change. I'm still leaning towards electrical.

Re: Early 70's 20hp outboard loses power at high RPMs

Get a manual for the motor. Seldoc has tables in the back for resistances. You could have a timing issue....check it out. It could be not advancing properly. The electrical system , including all grounds and wiring should be checked out. Also go over stop circuits for shorts. You're going to have to prove all electrical components are good then by default you may have a bad CDI. And, yes, there could be a bad coil.