to load up the crankcase with fresh fuel & then feed the cylinders with that load of fuel vapors on the downstrokes the crankcase vacum caused by the upstrokes must be strong enough suck in fuel from the carb.
if air seeps in from bad crankcase seals , the ” lowered” negative pressure needed to suck fresh fuel from the carb . will draw (suck) less fuel vapors …. less fuel =less energy to idle properly.
see video
(1852) Finding a vacuum leak – YouTube
something to look into after you check the easy stuff …
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