50hp Johnson problems

[M Kenyon]

I have a 1972 50hp johnson that will not idle very long before it gas fouls one plug (the top cylinder) and then just dies out. It will also run fine when under load or on the top end. It seems to clean out and fire the pug fine then. How do i determine if it's a problem with carbs or a reed valve. Thanks

 

[oldboat1]

Re: 50hp Johnson problems

The lower plug fires OK? no fouling on the lower cylinder?

 

[M Kenyon]

Re: 50hp Johnson problems

yes the lower plug fires fine and the motor runs fine at any throttle possistion other than idle the upper plug will also clean up and fire fine to when given more throttle. also when you slow down from the higher rpm ranges the motor run fine on both cylinders, but after it idles for short time one cylinder falls out it's always the top one

 

[oldboat1]

Re: 50hp Johnson problems

want to say dirty main jet, but my experience with dirty or clogged main jets has been opposite -- poor acceleration (or poor idling and poor acceleration). <br /><br />Wait for more opinion.....

 

[G DANE]

Re: 50hp Johnson problems

Hi M. Kenyon<br /><br />To me it sounds like your one carb is flooding, which will cause overrich mixture, specially at low rpm. At higher RPM it will burn even very rich mixture. Another reason could be a small leak in fuel pump diaphrame. You can test this by unscrewing fuel pump from motor, and with pump off, squeese primerbulb firm. If fuel drips from vacuum hole, diaphrame is leaking, into intake top cylinder.

 

[G DANE]

Re: 50hp Johnson problems

I forgot to ad: poor compression on upper cylinder would give incomplete combustion at low RPM too, leaving plug wet.

 

[M Kenyon]

Re: 50hp Johnson problems

I checked the float in the top carb and everything seems fine all the jets seem clean and the air bleeds seem to be clear also no dirt was found in float bowl. As for the float level i don't know were to measure it or what the spec. is. fuel pump seems fine and the spark at the plug is blue and looks very strong. also the compression is equal in both cylinders (150psi) if there is anything else to check please let me know.

 

[oldboat1]

Re: 50hp Johnson problems

You might try switching bowls and floats between carbs and see if there is any difference. I think the Clymer manual describes float adjustments, but absent a manual you could also compare the float in the top carb with that of the bottom carb.<br /><br />I haven't had any experience with reed problems, but that would remain to be addressed. Also, it's possible that the points, or one of the sets of points, needs to be reset (think that model has breaker points) -- or that the coil for the top cylinder is weak (although I think a coil problem would more likely be reflected in poor top end performance.)<br /><br />guessing....just guessing.

 

[almost retired again]

Re: 50hp Johnson problems

With the float bowl off, turn the carb. upside down the float should be parallel with the carb. body.