Swapped over to NGK GR4IX Iridium sparkplugs today in my daily workhorse (’73 Dodge Dart).
Was using Accel 416 sparkplugs before.
Sparkplug gap before and after was a little different;
Before with Accel 416 plugs: .026″
After with NGK iridium plugs: .036″
The Accel sparkplugs still looked fine as I had cleaned them up a few months ago. Didn’t notice any change during driving, but tried rechecking the idle advance.
The engine always liked to idle in Drive at 500rpm with 24 degrees advance, where engine load was lowest and engine vacuum was around 20-21″.
Now with the new NGK plugs, I found I could increase the idle ignition advance well upto 28-30 degrees and engine load would slightly decrease still (idling in Drive).
Could be the increased sparkplug gap simply cleaned up ignition for the engine and let run with more advance and less load.
But it never showed any signs of a hickup during idling. This engine could idle in Drive all day long at 500rpm.