I have a 69 350 with a Mallory multi-spark ignition system (i know i should change to MSD), Holley four barrel, hedman headers, and a thrush turbo exhaust. Everything was running fine until the humidity came back to Texas and know I get a pop from the right side header every couple minutes at idle and on deceleration. I checked the distributor and no signs of two cylinders firing at same time and there doesnt seem to be an arc issue with wires. Ideas???

As always, thanks in advance!

 

Did you check to see if you have any moisture under the distributor cap?

 

Yup. All looks good. No signs of moisture or more than one firing at same time.