1979 Johnson 100HP Half-Power

[benjhind]

I picked up a 1979 Johnson 100HP this season (100999S).

History: New to me this season. Previous owner had it out very little in recent years. Always had it professionally maintained as he wasn't the DIY type.

First few times on the water, it ran like a champ. Tons of power.

Now it has gotten moody. Most of the time, it still runs and idles perfectly. Then, like someone flipped a switch, it has very little power. When in "terrible mode" it won't idle, and won't throttle up very high in neutral. If I can get it in gear (by revving it up, dropping to idle and quickly shifting), it can barely get the boat on a plane. Then suddenly, it fixes itself and has full power.

Sometimes it starts out in "terrible mode" when cold starting, sometimes it starts out in "terrible mode" from starting while hot. Sometimes it does it while underway.

I'm scratching my head at this. It doesn't really seem fuel related, because the problem isn't consistent. I've had it out 4-5 times since it started and this keeps happening, but for the majority of the time it runs strong. I put new spark plugs in but haven't tried much else.

Help?

Ben

 

[interalian]

I'd be checking spark whenever it's in "terrible mode". Could be a duff coil or a bad power pack, or even the stator due to heat soak. If it consistently has no spark on one bank, swap the power packs from port to starboard. If the problem moves, there's your problem. Good luck!

 

[V153]

Ya it sounds more like an ignition problem vs fuel. Think I'd start by checking all the connections/grounds?

 

[benjhind]

Thanks folks!

Any ideas how I can check spark on such an intermittent issue? I'll try it on the muffs a few times and see if I can get it to happen, but it seems to only happen when I'm on the lake :/

Thanks again!
Ben

 

[interalian]

Hang an inductive timing light on each lead when it's acting up. See if you have spark.