Johnson 70 hp 1978 does not reach full speed/rpm

[Jonas_Vestlund]

I bought a small boat with a Johnson 70 hp from 1978 (two stroke, 3 cyl.) this Spring. The outboard engine has worked fine (nice sound, no problems starting etc) until now. The other day, I started the engine (no problems), let it get warm and slowly drove away from the harbor; the engine sounded fine and did not vibrate excessively or anything. However, when I, after a few minutes, wanted to accelerate, the engine felt powerless and refused to reach high rpm. Instead of reaching max rpm, the engine only reached, say 60%, stayed like that for 5 seconds, increased to 75-90% for a few seconds (without me doing anything), then down to 50-60% of full rpm again, etc. I thought there might be dirt in a fuel filter so I cleaned/exchanged both the filter on the fuel pump and the one of the fuel hose, without any improvement. Could it be the spark plugs (I tried to change them but they are totally stuck so I couldn't even unscew them)? -- but would the engine start so easily and run so smoothly if there was a problem with the plugs? Or could there be water in the fuel tank? The previous owner mentioned some problems with condensation in the tank -- should I buy one of those water-separating filters maybe? If there is water, do I need to disintegrate the carburator and remove any water from there? Greatful for any answer/help -- people with experience from Johnson outboards are scarce here in Sweden...

 

[JB]

Re: Johnson 70 hp 1978 does not reach full speed/rpm

Hello, Jonas.<br /><br />Welcome to iboats Bulletin Board.<br /><br />3 Cylinder outboards are notorious for starting and idling smoothly on 2 cylinders. I think that is what happened to you.<br /><br />By disabling the spark plugs, one at a time, you can determine if all cylinders are firing, and by pulling the lead wires from the plugs and holding close to the block you can determine if a dead cylinder has spark or not. (Handle the wires carefully. Rubber gloves recommended.)<br /><br />You must replace the spark plugs in any event. If they are seriously rusted into the head, remove the head and soak the inner parts of the plugs in penetrating oil for a few days. Replace the head gasket when you reinstall the head.<br /><br />If you do have to do that, you can do no harm by overhauling the carburetors and fuel pump.<br /><br />A firm "yes" to a water seperating filter.<br /><br />Good luck.

 

[Jonas_Vestlund]

Re: Johnson 70 hp 1978 does not reach full speed/rpm

Thanks a lot for your detailed reply, JB!! Yesterday evening, I managed to unscrew and change the spark plugs (using a little bit more force than I felt comfortable with...), and while two of them were covered in black, oily dirt, the third was noticably less dirty -- further supporting your hypothesis about only 2 cylinders working! I will try tonight whether they are firing, as you suggested. It's quite amazing that, just hours after I post a question from Sweden, I get an excellent reply from the other side of the globe, from Whitt, Texas!