1975 70 hp Johnson intermittent starting

[salmontales]

My 1975 70hp Johnson starts just fine if it’s been sitting awhile – several days. It also runs fairly smoothly – for a 30 year-old 3-cylinder. If I run it in gear at a relatively slow speed (6 mph – no tachometer) for 15 minutes, stop it for 15 minutes, and try to start it again – it won’t start. It turns over just fine, but absolutely does not fire. This has happened four consecutive times following:<br /><br />1. visual inspection of the control box and re-wiring of the battery to a battery selector switch – the electrical connections to the battery are good,<br /><br />2. replacement of the spark plugs – they were dark and wet (I did a fair amount of slow speed trolling, but have since added a trolling motor in order to save the 70 hp), <br />3. replacement of the fuel line and fuel pump, and<br /> <br />4. replacement of the power pack. <br /><br />I tried to replicate this problem in the driveway with rabbit ears and no luck – it started right up.<br /><br />I hired my local certified Johnson mechanic for a maximum of $145.00 to try and diagnose the problem and no luck – they ran it for 1 hour in a test tank, let it sit for awhile and it started again. They recommended the power pack. They also noted that they thought – but were not sure – that the motor tried to start itself while sitting in the yard; and recommended a new ignition switch (which I haven’t replaced yet).<br /><br />Because this only happens after running in the water, I haven’t really tried checking the spark when it doesn’t start, since it’s usually at dock-side. I suspect I need to take it down to the local launch, run it, confirm it won’t start and take it home to work on it immediately – but I’m not quite sure what I’m looking for.<br /><br />Another possibly related symptom is that I have a fair amount of electrical noise on my fish finder and the dealer suggested a new rectifier.<br /><br />I haven’t been able to find a description of this problem in the Clymer manual troubleshooting guide. Any sage advice would be greatly appreciated – fishing season is coming soon!

 

[OBJ]

Re: 1975 70 hp Johnson intermittent starting

Welcome to iboats salmontales!<br /><br />One thing that jumps to mind is the difference between running in the drive way or in a test tank. The boat and motor are stationary. Not moving. On the water with the boat in motion and going through swells or waves, the boat and motor are getting bounced a little. And since it seems the problem only occurs on the water, I'd take a look for some lose connections or even possibly a broke/frayed wire in the harness that could be shorting out the ignition system. Try shaking the wire harness around a little with the engine running and see if it stalls or quits. Seems most of the other bases have been covered.