25 HP Johnson Motor Spark Plugs Issue.

[shabu03]

I have a 25HP Johnson model number 25R77S. I'm having issues with the spark plugs turning black after running the motor for 15 minutes. Replaced the carb last year. The adjustment is about 1 turn out for the motor to run in the lean positon. Motor starts up fine, after about 15 minutes it starts to act up. 1st question is what are the correct spark plugs for this motor, also the gap? Went to a marine store and they sold me NGK and told me to not worring about gapping the plugs. Would not enough spark from the coil create the plugs to foul ? I'm confused on what to do.

Thank you.

 

[Vic.S]

Re: 25 HP Johnson Motor Spark Plugs Issue.

The recommended plugs are Champion QL77JC4 or QL82C gapped at 0.030"

The latter are a little hotter and some people prefer them for slow speed running eg trolling

The Q denotes inductive suppression. If suppressed plugs are not required L77JC4 or L82C would be acceptable.

You do not say what NGK plugs you were sold.

The nearest NGK eqivalent is BZ7HS. B7HS is the non suppressed version.

DO NOT use resitor plugs with OMC CD ignition E.g. BR7HS, often sold and even listed as equivalent to the Champion "Q" series ... they are not !

NGK are also sold pre-gapped. -8 ,-10 etc after the number indicates the gap in tenths of a mm so -10 is pre-gapped 1.0mm, approx 0.040"

Set your gaps to the correct figure!

See HERE for full instructions for adjusting the mixture needles.

 

[HighTrim]

Re: 25 HP Johnson Motor Spark Plugs Issue.

What mix are you running?

 

[shabu03]

Re: 25 HP Johnson Motor Spark Plugs Issue.

50:1 is the mix that I'm using.

 

[HighTrim]

Re: 25 HP Johnson Motor Spark Plugs Issue.

Are they black with some light soot on them? You may have a restricted air flow, or a too rich fuel mix. The black plug is often related to low compression on that cylinder as well, I would check it out.

Start by testing the compression, then ensure that your slow speed needle is adjusted properly as posted by Vic.

 

[F_R]

Re: 25 HP Johnson Motor Spark Plugs Issue.

Black is normal. "Reading" spark plugs went out the window with the introduction of CD ignition. The plugs do not get hot enough to burn off the smudge like in days past. The ignition is supposed to be able to fire through all that gunk.