1978 Johnson 70hp stopped running on the lake

I'm not new to boats but I am new to doing the repair work myself! I bought a 70hp 1978 Johnson, tried it on the hose and it ran great. So I took it out for a test, mostly to make sure the boat that came with it wasn't going to sink. I ran on low speed for about 10 minutes, shut it down and dropped anchor to check out various things on the boat itself (ok, I mostly made sure that my fishing pole was compatible with the boat). Anyway, after about 10 minutes I tried to start it again and it wouldn't start. Tried with the choke, without the choke, made sure the tank was ventilated, pumped the ball, let it sit in case I flooded it, took off the fuel filter and made sure it wasn't plugged, then ended up rowing back to shore.

When I got it home I made sure the spark plugs all had spark. Not sure what else to do except take it to a service shop, but I really don't wanna!

Probably an easy answer, but why would it just not start again after running perfectly 10 minutes earlier?

Thanks!

Re: 1978 Johnson 70hp stopped running on the lake

Did it have spark?
It is possiable for an electrical component to overheat and cause a problem, and when it cools off it works properly.

Re: 1978 Johnson 70hp stopped running on the lake

It had a spark. I tried again much later in the day but still no luck.

Re: 1978 Johnson 70hp stopped running on the lake

All it takes is compression, spark and fuel. What are you missing?

Re: 1978 Johnson 70hp stopped running on the lake

That's a great question! I was hoping there was an easy answer, like "Oh, just make sure you pump the so and so and blow the dirt out of the whatsit". But it's really a head scratcher for me.

I took off the carb cover and shot some ether in there, didn't even hit. Cleaned the plugs, they were all wet so gas is getting in there, and they all had a spark. I just don't get it. Is there a kill switch that might be stuck? Maybe the problem is in the ignition switch somehow?

Re: 1978 Johnson 70hp stopped running on the lake

How are you checking spark, with an air gap tester or hloding the plug against the block? If you had spark it should pop over on either. By the way, don't use either in your engine, it has no lube in it and can cause problems. Squirt some fuel mixture into the carbs instead.

Re: 1978 Johnson 70hp stopped running on the lake

Have you done a compression test? A crude but effective way to do it is to put your thumb over the plug holes and have someone turn the key. If your thumb gets blown off with good pressure you'll know you at least have compression even though you won't know how much.

Re: 1978 Johnson 70hp stopped running on the lake

Be careful, you might also find out if it has spark at the same time.

Re: 1978 Johnson 70hp stopped running on the lake

Daviet said:

Be careful, you might also find out if it has spark at the same time.

Click to expand...

Please elaborate. I need to know if I need to tell my friends that own boats to quit doing this test. No one likes getting the pee shocked out of them.

Re: 1978 Johnson 70hp stopped running on the lake

I'm holding the plug against the block and seeing a spark. I'll try a compression test (and be sure to unplug the spark plug first!) but I'm sure there is compression.

Re: 1978 Johnson 70hp stopped running on the lake

If spark and compression are known to be good and it is getting fuel, there is a possibility that it sheared the flywheel key.

To check that, you will have to pull the fly wheel off with a fly wheel puller and check the key. It is used to hold the flywheel at the correct position on the crank so that spark occurs at the proper time.

Hope this helps.

Re: 1978 Johnson 70hp stopped running on the lake

Please elaborate. I need to know if I need to tell my friends that own boats to quit doing this test. No one likes getting the pee shocked out of them.
That's exactly what I mean, you can get a pretty good jolt out of that ign system.