1979 Johnson 70hp loss of spark on one cylinder

[Lachlan]

G'day, recently brought a 70hp 1979 Johnson and took it out for the first time. Boat was going at what I thought was top speed (about 10-15mpi) for about 20 minutes then suddenly it jumped up to about double that, top speed roughly 30mpi.

Gave it a good run for 15 minutes then anchored up for an hour, moved fishing spots and it ran just as well then anchored up again and the third time it ran at top speed for a little bit then back to about 10-15mpi.

Still starts, idles and runs fine just doesn't go very quick.
Checked the spark plugs and the bottom cylinder spark plug isn't sparking at all, I swapped the bottom coil to the middle and it worked so I know the coil is fine.

​I thought it was odd that it ran on 2 cylinders then I assume the 3rd started firing so it was running good then back to 2
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Was hoping to get some opinions on if this is likely to be a power pack or stator problem? Is there a way to find out without taking the fly wheel off?

Any help is much appreciated

 

[racerone]

Yes you can test the components under the flywheel without removing the flywheel..-----I would think it is a powerpack issue.------See if you can make #3 sensor coil fire #1 or #2 coil. by swapping sensor wires around in the rubber plug in..

 

[Lachlan]

I tried plugging #2 coil into the #3 sensor in the power pack and the spark plug end into cylinder 3 and it did not fire, is that what you mean? Sorry if i don't understand i'm new to all of this.

Thanks for the reply

 

[racerone]

Sensor wires come from under the flywheel.-------They can be swapped at the rubber plug in.------Or just try a new powerpack as they are not that expensive.

 

[Lachlan]

Ah right i understand now, ill try that tomorrow, thanks. Yeah that was plan B but wanted to know if i was missing anything obvious before buying one