1981 Johnson 40 Commercial. Runs ruff, won't idle?

m_sailer

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Feb 27, 2013
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Hey there all, hoping someone can point me in the right direction. Bought an '81 Johnson 40 commercial tiller engine and haven't been able to get it running smooth. Seems to start up quick but skips every 5th beat. It will stay running and rev up when throttle is applied, but dies soon after throttle down. The top spark plug is black and sooty, bottom plug is dark and wet.
Hope someone can help me draw an informed conclusion based on these symptoms. In the mean time, I have a carburetor
kit, condensers, and points on order... thanks for your time!
 

m_sailer

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Re: 1981 Johnson 40 Commercial. Runs ruff, won't idle?

Update: Spark tester seemed to show spark on both cylinders. Motor runs on either cylinder by itself. That is, if I unplug either spark plug the engine will still fire up. Pumping the ball has no effect on the misfire/stuttering. Is there a way to test voltage coming from under the flywheel before the second coils? Don't want to pull the flywheel off if I can just use my multi-meter... Thank you for any info!!! Much appreciated!!
 

Silvertip

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Sep 22, 2003
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Re: 1981 Johnson 40 Commercial. Runs ruff, won't idle?

More than likely a gummed up carb priblem. You have the kit. Use it.
 

m_sailer

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Feb 27, 2013
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Re: 1981 Johnson 40 Commercial. Runs ruff, won't idle?

Thank you Silvertip for your input. After a thorough cleaning and replacing the parts from the kit the engine runs much better and idles better. However the miss is still there.

I will go ahead and order the points and condenser kit just for the peace of mind.

My buddy thinks it may have something to do with the hoses that bring fuel from the bottom of the intake to the sides of the cylinder? Anyone else second that?

Thank you for any input!

Matt - 10 years marine electronics installation, plumbing, and detailing
 

m_sailer

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Re: 1981 Johnson 40 Commercial. Runs ruff, won't idle?

Update:
The carburetor was pretty clean. The float had been tweaked down considerably, definitely not parallel with the bottom of the carb. Could this have been someone's attempt to lean out the mixture (due to the sooty and fouling plugs)?

Removed the flywheel and replaced the points and condensers. Can anyone verify that the stop/kill button should be wired to the same node as the condensers? Set the point gaps to .020. The old points were a little closer together, about .015'ish. Again, the engine runs better now, even runs smooth for 20 seconds or so, then starts missing...

My old Johnson shop manual calls for l77j4 spark plugs, I purchased the l77jc4 equivalents... still no joy.

Someone suggested rich mixture due to the fouled and sooty spark plugs. I find removing the fuel line while running while running seems to help it run steady until it dies.

Getting frustrated, hope some kind soul will read this and point me in the right direction. Thank you all for your help!
 

m_sailer

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Feb 27, 2013
Messages
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Re: 1981 Johnson 40 Commercial. Runs ruff, won't idle?

update:
tried tweaking the carb float down 1/16", now runs smooth with %50 choke...
 
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