Check that the blade belt is engaging and disengaging correctly.
With the blade down, the engine is under load. When you lift the
blade, it is supposed to turn the blade off, which would take the
load off the engine. If it is working correctly, then you probably
need to either adjust or clean the carb, or if it is a Tecumseh,
you may need to adjust the governor, as they can loosen up occasionally.

Matt
30 years in Outdoor Power Equipment repairs.

the blade is disengageing or it wouldn't be trying to run on the idle
circuit which is obviously lean. a misadjusted governor would run too
fast or too slow.
start with the fuel system where the problem lies.
chip.