If the engine has high miles, there might be enough carbon deposits in the combustion chambers to cause detonation regardless of where the ignition timing is set. Once carbon deposits get hot enough to glow (just a few minutes' warmup) as soon as the engine lugs a little on an uphill pull the glowing carbon will light off the fuel/air mixture. The spark plug comes along later and tries to light a fire that's already burning.

Fill a 1-liter soft drink bottle with HOT water, take the air cleaner off, set the idle up to around 1500 RPM, and slowly dribble the whole bottle down the carb throat. The engine will buck and shudder, and clouds of steam and possibly huge gobs of carbon will get blown out of the tailpipe. Don't pour fast enough to stall or hydraulic-lock the engine, just give the combustion chambers a good steam cleaning. Then set the timing to factory specs and take a test drive at full throttle up a long hill. You'll either fix it or you won't, but you won't do any harm.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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