[drolston]

Check for a nice blue spark that will jump 3/8" at idle, for each plug. Get it idling some dark night with the garage lights off and look under the hood for sparking along the ignition cables. Worn/cracked/soggy ignition cables may allow cross firing and backfiring. Too big of a spark plug gap will aggravate that problem. Set all at 0.025" Check that the spark advance adjustment on the distributor is near centered.