johnson 4 hp

[kieronriley]

hi ,and good evening, a lad in our club has just bought a johnson 4 hp outboard and it didn't run properly, so he asked me if i would clean his carb as that's what he thought it was. i cleaned the carb(sonic cleaner) and put it back on but it was no better, on further examination i found that the bottom plug wasn't firing so i put a new plug in it and it still didn't fire (but there was a spark now, then on playing a bit more with it i discovered that when it reached almost full revs then the bottom cylinder stared to fire up and it revved really fast, so my thoughts are that there is not enough compression but as the piston speeds up so the compression builds up and it fires, what do you think. I have a compression tester being delivered on Thursday so i hope that will help me with the diagnosis . Any thoughts are really welcome ,thanks. Kieron

 

[rotrax]

I have two of these, punchy and smooth running when working properly.

One of mine is working properly, the other is like the one you describe.

I suspect the bottom crancase seal. I shall attend to my one this winter perhaps.

 

[VicS]

hi ,and good evening, a lad in our club has just bought a johnson 4 hp outboard and it didn't run properly, so he asked me if i would clean his carb as that's what he thought it was. i cleaned the carb(sonic cleaner) and put it back on but it was no better, on further examination i found that the bottom plug wasn't firing so i put a new plug in it and it still didn't fire (but there was a spark now, then on playing a bit more with it i discovered that when it reached almost full revs then the bottom cylinder stared to fire up and it revved really fast, so my thoughts are that there is not enough compression but as the piston speeds up so the compression builds up and it fires, what do you think. I have a compression tester being delivered on Thursday so i hope that will help me with the diagnosis . Any thoughts are really welcome ,thanks. Kieron

Yes do a compression test. The readings should be within 10% of each other
Also check the strengths of the sparks using an adjustable air gap tester (improvised if necessary).
You have not given the model number so could be points or cd ignition. You should get a spark which will jump 5/16 inch from points and 7/16 inch from cd.

 

[Steve65]

Sounds to me like a coil breaking down...common on these engines

 

[fushion julz]

I have one of these and have had several more, until recently. Pull the flywheel and check points gap...If that is OK, then change coils and condensers... I've had HT leads go bad in the past, too

 

[VicS]

Sounds to me like a coil breaking down...common on these engines

Yes That is why I suggested checking the sparks but it depends on the age and exact model . I'd not expect a model with CD ignition to have trouble with the coils but one with points ignition could well have, especially an older one

If the OP posts the model number we will know exactly what we have. Points or CD

 

[kieronriley]

hi and sorry for the delay in coming back. the serial number is J4BRACTA its coils and points and despite the both coils looking new still all nice green paint and no rust or signs of cracks,i did today remove number 2 coil an underneath it where arcing marks and there was an actual hole in the bottom of the coil, so I've ordered two new ones today and will let you know the outcome when i fit them.

 

[VicS]

hi and sorry for the delay in coming back. the serial number is J4BRACTA its coils and points and despite the both coils looking new still all nice green paint and no rust or signs of cracks,i did today remove number 2 coil an underneath it where arcing marks and there was an actual hole in the bottom of the coil, so I've ordered two new ones today and will let you know the outcome when i fit them.

That makes it a 1983 European (Belgium) made model but I think it should read J4BRHCTA where H indicates that it does not have an integral fuel tank.
When you finish besure to torque the flywheel retaining nut correctly.