For my 1998 ultra with just a PC V and big sucker what would the gap be set at for the NGK iridium plugs?

 

Should still be .040. You have to be careful not to damage the fine wire center electrode.

 

Came in at .039 so I'll leave them there.

 

If you were running Champions the gap would be the distance you can throw them away from the bike.

carl

 

Is that kinda golf and then you go look for it

WP

 

I was told by a builder that .038" was best for performance...

 

Not quite the whole story. It depends on how the engine is set up as well as load and environmental factors.

For example in cooler weather you can run a slightly larger gap for bit more crisp response and a fraction more power. In the summer, you can't do that and your gap needs to be slightly narrower to combat detonation.

That is just one over simplified example. Things like compression, ignition timing, air fuel ratio, weight of the bike with the rider, barometric pressure, and other factors come into play that are beyond the scope of this thread.

The factory spec is the best and safest bet for most conditions. Some gains may be attainable with a larger or smaller gap, but only when you are building bleeding edge performance and account for risk / longevity.


Long ago I was into all that stuff. Needless to say my Sportster I had at the time was a fairly brisk riding little bike.

 

Our outstanding Mods in action...... TY

 

Love the NGK Iridiums just as they come out of the box. Never tried to gap'em.

 

EVO had a wide range 0.038 to 0.043. And years of riding one proves that anywhere in between makes no difference what so ever.