I had way too many problems with the D16's.
Mostly it was excessive fouling as the plugs run too cold. In the summer it seemed ok, but the first time we got a cold snap, Ellie wouldn't start - period
I double checked everything I could think of, cleaned the plugs, checked the gaps, checked the rotor, condenser, points, cables, -- you name it, I went through it.
Then one of the guys asked me which plug I was using -- aaaaahhh DUHhhh D-16's
. Went to APM, couldn't get D-21's, or at any other parts house, so got the NGK equivalents. TaDa
Ellie started first pop! Sooo, now I use equivalents. I am hoping to find some D-21's one of these days.... but the NGK's work really well. No more fouling either!
Hope that explains it a bit.