I own an 87 Johnson 20hp,Just rebuilt the carbs,set the low-speed needle 1 turn out from lightly seated,ran fine all day,after idling back to dock for a while,it stalled and was hard to get started.I got home and checked out the plugs(L78V).They both were healvily coated with carbon and were shiny like they were wet.My manual calls for QL77JC4.Do you think i should replace the old-ones with the JC4's or put the L78V's back in there?