Accel plugs 416S

#3 look’s different from others. Have you run a compression check?

Compression on #3 looks consistent but something seems off on that one. Bad plug wire? Valve guide seal?

How old were plugs when pulled? Lots of street miles or just a couple 1/4 trips?

Do you have a compression tester with a separate rubber hose that goes to the gauge ?

Heads are brand new Dart Aluminum. Put on late last summer. Plugs and heads have less than 1000 miles on them. New rebuilt engine maybe a couple 1000 miles on it. Maybe plug wire? How about a faulty plug? Have read on here about Accel plugs being bad right out of box. Car seems to run fine, no miss or anything. Mixed plugs up so #3 plug is now on other side of engine. Will check again after a few miles. Compression tester does have a separate hose but more along the lines of hooker header tubes are in the way. Thought about fabricating maybe a 90 so compression tester will work but just haven't gotten around to it yet.