If you don't happen to have a Sun distributor tester handy (I do) An Innova delay flash timing light will turn your engine into a field-expedient tester, and it will also tell you if the vacuum advance is working properly. Definitely start out with a thorough disassembly and cleanup of the distributor, and pay particular attention to the amount of shaft side play where the shaft exits the top of the housing. I install a Torrington needle bearing running against a hardened steel sleeve at the top and check to make sure the cam lobe lift and spacing is exactly correct. It's not uncommon to have a few degrees of error from one distributor cam lobe high point to another, and/or a few thousandths of difference in point gap (dwell) between cylinders. That equates to ignition timing errors from one cylinder to another.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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