1970s Johnson 20hp fouling plug

[Steve Mahler]

I have an early 70s Johnson 20hp outboard which generally runs great except for a couple problems:

1) After running for even an hour or less, one of the plugs becomes fouled. This is independent of engine speed - even high speed extended runs leave one plug fouled.
2) Because of this I am constantly changing plugs, literally one set a day if fishing on and off etc, since it has trouble cold starting with one fouled plug.
3) Even with new plugs, I have trouble cold starting the engine. 70 degrees + = two pulls max. 60-70 deg F = 4-5 pulls. Below 60 get out the paddles.

The engine runs great in all RPM ranges - idle to WOT with no smoke or hiccups. I had the carb prof. cleaned this spring, but I am wondering if my spark is not hot enough (coils?)

I just got the manual on CD on Ebay, and I just got a compression tester. This machine has been in the family since new, never left out in the rain....

All pointers appreciated - would love to know the first step in diagnosis. (i have a heated workspace to mess with it over the winter)

 

[tashasdaddy]

Re: 1970s Johnson 20hp fouling plug

compression test, and spark test. post results.

 

[JUSTINTIME]

Re: 1970s Johnson 20hp fouling plug

visual inspection
spark test
compression test
fuel samples

 

[DJ]

Re: 1970s Johnson 20hp fouling plug

First off, let's make sure your engine is just ill and not terminal.

A compression test will tell us that. Let us know.

 

[steelespike]

Re: 1970s Johnson 20hp fouling plug

On some motors if the fuel pump diaphram has a hole in it it will foul a plug.
Check the spark.It should be blue and you should here it snap.If it is on the original coils there is a good chance they need replacement.Also the points probably need cleaning adjusting and or replacement by this time.