a lean mix is very much more powerful than a safely slightly rich mixture.. burns hot and fast.. that's why an engine's power surges when it runs out of fuel..
plug chops are running a new plug, killing the engine and examining the plug.. search "plug reading" on the net to learn how to read plugs... and/or search this forum for the complete plug chop procedure.
But plug chops won't reveal much if the problem is a malfunctioning carb. They are a tuning aid and results will be misleading if the engine's got problems.
get down and remove the carb.. disassemble it and if necessary clean it.. repair gaskets or whatever so it functions as it did when new.
This chore should include the air intake system as well as fuel flow through the entire fuel pathway, from gas cap vent to the engine.. And it includes cleaning all screens / fuel filters and curing any vaciuum leaks in and around the intake manifold and cylinder.
just get the thing back to as-new stock condition..