spark plug recommendation

[Bored&Stroked]

You'll find that with most of today's gas, that plugs don't "color like they used too". It is difficult to achieve a nice chocolate brown color . . . just never seems to happen for me. I tend to see colors that are a much lighter brownish/grey when the mixture is right.

As RoTo mentioned, you should do a test on the highway . . . is the only real way to do it. If you drive around, then come to stops and idle the engine . . . you'll find they almost always look too rich. So, you can't check the plugs after you go home - you'll not have accurate coloring . . .

Or, put some O2 sensor bungs in your exhaust and use a meter to tell you what is really happening.

[3twinridges]

I had a similar issue at cruise speed, good advice on the plugs. In my case, the driveshaft was severely out of balance. Had a driveline shop balance it and all good.

JB

[farmertom]

anyone using autolite 386s? seem to work fine in my orig 37 flathead.

[svmsr1044]

Ronnie, I’m curious why you recommend a larger plug gap for 12 volts. Is the secondary voltage of a 12 volt system higher than a 6 volt system?