Carbon in the exhaust- - - - -too rich. Lean it out at least halfway back to the original mixture. Find a LONG hill where you can run full throttle at normal cruising speed for 1/2 mile or more, without overspeeding the engine and listen for detonation- - - -"spark knock"- - - -whatever term you choose to use. It sounds like BB's rattling around in a tin can. Retard the timing a degree or two and repeat the uphill pull. Keep doing this until the rattle goes away, or at least can be stopped by easing up on the throttle position a little. That's what the "octane selector" was originally designed to do- - - -allow a driver to tweak the timing a little to compensate for differences in altitude and/or differences in the grade of gasoline from tank to tank.
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
Love your enemies and drive 'em nuts!