Determining whether spark plugs are still good

[PPP]

My '77 Mercury 850 w/Thunderbolt ignition had NGK plugs #BUHW when I acquired it.. As you probably know, these plugs do not have a gap as the older plugs did. Is there a way, other than the motor functioning well, to determine if the plugs are firing properly? The motor is slow to start, then seems to run rough in my back yard with muffs on.

 

[Sunfish12]

Re: Determining whether spark plugs are still good

You don't say when you acquired it. Pull the plugs and check for carbon buildup, rust, and wear.

 

[j_martin]

Re: Determining whether spark plugs are still good

They're only a coupla bucks apiece. Throw in a new set and see if it helps. If it does, throw the old ones. If not, they're good spares.