1977 Johnson 70 hp cutting out got a question

leesumm

Petty Officer 3rd Class
Joined
Jul 22, 2012
Messages
84
This just started yesterday running across the lake. I left the dock and everything was fine. I stopped at a spot and fished for about an hour. Wind came up and I started for the other side of the lake , after a few minutes the motor started to misfire (for lack of a better description) running about 1/2 throttle. I stopped and fished a while and pondered what could be wrong. Wind came up and I started for the other side of the lake, ran good for about 5 minutes then same thing . Kinda souinded like running out of gas. I filled the tank before leaving home so I knew I had plenty, I checked the choke to make sure it was open, Ok, I checked the vent valve its open. Made it to the other side and sat and fished for a while, decided to go home. Started and ran fine for about 5 minutes , same thing. I throttled back and creeped along , ran fine. Took the screen off the fuel filter and its ok, So now where do I look and what for?
 

Home Cookin'

Fleet Admiral
Joined
May 26, 2009
Messages
9,715
Re: 1977 Johnson 70 hp cutting out got a question

I just chased a similar problem with a 1987 70. What makes it hard is that you have similar symptoms that can be fuel or electrical. You have to go along eliminating issues, and you need to run the boat in open water, and you need someone with you. And you have to wait for it to do it again.

Fuel: you tried the choke, no effect. try pumping the ball, too. run from a remote tank. drain the carb bowls. My mechanic went out with me, removed the cover over the carbs, and sprayed a windex bottle of mixed fuel in each one (when the motor acted up) and when that didn't correct it, we knew it was electrical.

Electrical: figure out which cylinder is missing. Could well be two: they run OK on just two and crap out on just one. A spark test at the dock doesn't always work b/c it's not running at higher speed under a load. It's hard to pull wires while running but that's the drill.

Replace plugs; replace wires. Check and reconnect all connections, especially grounds. unbolt, clean and rebolt the coils (in case they have lost their ground).

I was able to test a new power pack and when that wasn't it, return it; most places won't let you.
Once we figured which cylinder was going out (top) , we swapped the coils (top and center). The problem followed the coil. Put a new coil on; it helped but still had one dropping; put a second coil on and so far so good.

Coils when they get old will sometimes malfunction when they get hot.

you can look for shorts by runnnig it at night with the cover off and look for sparks

Note: I am making a terrible assumption that your 77 works like my 87. And I'm no expert; my feelings won't be hurt by any who are and who say my advice is wrong.
 

leesumm

Petty Officer 3rd Class
Joined
Jul 22, 2012
Messages
84
Re: 1977 Johnson 70 hp cutting out got a question

ok I have no one to ride with me that can drive the boat. Last summer I changed all plugs. Pulled all carbs and cleaned them. Replaced all fuel lines, did a spark check everything was normal then. It just started out of no where for no reason. I idled back and it seemed to run fine , it was when I was running about 1/2 throttle which I usually run at . Guess I am going to have to go further with it now
 

Home Cookin'

Fleet Admiral
Joined
May 26, 2009
Messages
9,715
Re: 1977 Johnson 70 hp cutting out got a question

try choking it when you run and it starts acting up. If no effect, it's electrical.

Try a remote tank with the bulb near you to squeeze.

Run without the cover. When it starts acting up, stop where you are, pull a plug wire, restart--same effect? it's that cylinder. Worse? it's not. try all three, one at a time.

When you find which cylinder is dropping, work on its connections; if that doesn't work, start swapping coils. Since coils are only about $30 carry a new one and put it on the bad cylinder.

The pain is that you can't duplicate the problem in a shop or at a ramp/pier.
 
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