There is one hose from the intake.It looks good. I used carb cleaner to spray around all hoses and carb and I detected no signs of a leak anywhere while idling.
My point is it may not be a leak, but a malfunctioning emissions
system. You wouldn't detect any leaks, it could just be sucking
air from somewhere in the system when it's not supposed to.
I don't want to send you on a futile hunt though. I would just
study the intake system from the service manual, if there's
enough information there to do that. See what other systems/
parts are connected to it, and when they are supoosed to be
on or off. If you are not sure, you could even try removing
the hose you mentioned from the intake, and blocking off
the hole. If that helps, find out what that hose is supposed to
do.
BTW Have you tried replacing the spark plug?