I've seen this symptom repeatedly on engines with carburetors and mechanical fuel pumps- - - - -surging on acceleration due to low fuel pump output or a clogged fuel filter, and just barely able to maintain steady RPM on a light-throttle cruise. A slight increase in fuel demand drops the float level, and a lean surge develops. The float bowl doesn't have to get empty, just a drop from the normal level of the fuel in the bowl in relationship to the venturi makes the mixture go lean.
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!