Reply To: 1972 Mercury 20 hp

[David Stanton]

How did you check for spark? Spark jumping the spark plug gap in air under no compression usually means you have good spark, but if the voltage is weak, it may not fire well under compression. You should be able to make the spark jump 1/4 to 3/8th inch in free air. (some like to see 1/2 inch). You can make a test jig to do this, or I do the next best thing, taking an old spark plug and prying the gap wide open.

Dave