JOHNSON 70HP LACK OF POWER

[vision2000]

I just bought a Johnson 70hp outboard , 1980 model, 3 cylinder, 2 stroke. I used it for an hour and it was perfect. After trying to use it again after about 2 hours, it worked fine for about 5 minutes, than I had a lack of power problem. With full trottle it only gave me 5 knots whilst before it gave me 25 knots. Fuel line is all new including tank. Just serviced with new carborator gaskets. Your help is deeply appreciated as I am going mad. Thanks

 

[DJ]

Re: JOHNSON 70HP LACK OF POWER

You are most likely not getting power from all three cylinders.

First, check spark on all three.

What do the spark plugs look like?

 

[F_R]

Re: JOHNSON 70HP LACK OF POWER

Most likely you it is not running on all 3 cylinders. There is a standard routine for checking things such as this out.
1. Check the compression You could have internal damage and nothing external will ever fix it.
2. Check the spark on all three wires. Should jump a 1/2" gap with ease.
3. Chances are it's just spark plugs. You can't tell by looking. If any doubt at all, try new ones,,,And the correct type!!!! Just because they look cruddy makes very little difference on today's motors with powerful CD ignition and super-cold heat range plugs and no longer dumping unburned fuel overboard.
4. Check to see which cylinder is not firing. Remove and ground one spark plug wire at a time and run it and see which one makes no difference. That is the one you need to fix.
5. Assuming the missing cylinder passed the first tests, then, and only then, check the carb associated with that cylinder. Most likely a clogged high speed orfice jet. They have to be very clean...NO gum or crud in them.
6. OR go ahead and bypass all the steps, assume "that's ok" and tear you hair out.

 

[vision2000]

Re: JOHNSON 70HP LACK OF POWER

Compression has been checked. All 3 pistons have good compression. Spark has been checked and all is OK on all 3. Spark plugs are ok also ( new ones ). Could it be an intermitant electrical fault ?

 

[F_R]

Re: JOHNSON 70HP LACK OF POWER

OK, you have made most of the suggested checks, now you'll have to isolate which cylinder is not working, while it is not working, then zero in on that one. Don't let your imagination run wild.

 

[xtraham]

Re: JOHNSON 70HP LACK OF POWER

try bumping the choke tell us if it dies out or picks up speed, do this @ about half throttle, in the water...

 

[vision2000]

Re: JOHNSON 70HP LACK OF POWER

Hi All. I think I found the problem - it's fuel starvation.
Due to the fact that I fitted a new plastic fuel tank, I made a hole to fit the fuel guage. It seems that I left some plastic granules in the tank because I found the fuel pump nearly totally clogged with these plastic granules. Now I cleaned all up, fitted a seperate fuel filter and will give it a test tomorrow. Thanks alot for your help - will keep you posted.