1984 Johnson 150hp GT No power

amvaneyk

Seaman Apprentice
Joined
Oct 12, 2010
Messages
36
I have a 1984 Johnson 150 hp outboard that idles well and starts up on the first try.
When I put her in gear and open her up right away she chokes out and dies. However, if I run it at a slow speed (2-5mph) for a while I can eventually open her wide and she will do 40 mph with no problem. I had a compression test done on her and that came back fine. On Saturday I had all 6 carbs cleaned and rebuilt and I ran sea foam through her. I took it out to the lake and it started right up and I was able to go from neutral to top speed with no problems. About 30 mins later however, the same problem occurred. If you have any ideas I would love to know. Thanks!
 

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: 1984 Johnson 150hp GT No power

Sounds like you took care of the carb issue. It's possible you still have an ignition issue. Weak ignition components tend to fail when the engine heats up to normal operating temps. Take an inductive timing light to the lake with you next time out. When the engine starts to slow, pull the cowling and check each of the 6 plugwires for spark. The flashes in the gun will show you what's going on with the spark to the plugs. That engine will seem to run fine with 4 or 5 plugs firing, yet make no hp...
 

amvaneyk

Seaman Apprentice
Joined
Oct 12, 2010
Messages
36
Re: 1984 Johnson 150hp GT No power

Ok I will try that and report back. Thanks! Even though it idles and revs fine while on a hose in my back yard could it be only using a few of the plugs? If so could I just test them with the light in my backyard, or do i need to bring it to the lake?
 

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: 1984 Johnson 150hp GT No power

Yes, you can check it in the backyard on the trailer. The issue may be how long it takes the ignition components to heat up to normal operating temps. If you run the engine on the garden hose, it may run cool, based on the incoming water temps.....lakewater is typically warmer this time of year than the household water. Also, it's good to test the ignition when the engine is under load. Floating in gear at idle places load on the engine-which you can't duplicate on the trailer.
 
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