sparkplug gap size

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bairdco

a guy who makes cool bikes
Aug 18, 2009
6,537
264
63
living the dream in southern california
gap's the same. i run my 66 at .030

i've seen instructions saying .036, but that's a little wide.

for experimental purposes, i ran every gap from .010 (barely started) and went .005 increments all the way to .055 (barely ran,) and found it best overall at .030.

between .025 and .035 there was no noticeable difference.

your plug should be a nice cocoa brown color.

light tan or white is too lean and running really hot, while a dark and oily plug is too rich and not burning all the fuel.
 

DaveC

Member
Jul 14, 2010
969
1
18
Boise, ID
I just installed a NGK BR9HIX Iridium plug. Said on the bill of sale to not gap the plug. It comes preset at .031. Apparently the center Iridium post snaps off if you try to gap it.

At $15 each shipped I'll not be setting the gap :p
 

bairdco

a guy who makes cool bikes
Aug 18, 2009
6,537
264
63
living the dream in southern california
that's kinda what i was experimenting with.

i know from race car experience that a smaller gap is better at higher RPM's to stop misfiring, as you want a quick spark to keep up. a larger gap takes longer for the spark, and although your car will start easier and idle better, it suffers at speed.

there's another older thread around here about gaps, and i detailed my experiment more.

basically, i startd at .030 and kept going down .005 at a time till it wouldn't start (which was .008)

then i went back to .030 and went up to .050 or something till it would barely run.

overall, it worked fine between .025 and .035, so i just split the difference and went .030