If this eng has a split intake manifold,2 pc held together with rivets,before condeming carb carefully inspect rivets that hold manifold together,look for loose/broken rivets,if none found then take a can of spray carb cleaner with ext tube & slowly spray around joint while eng running slightly above idle,if eng speed changes while spraying then either gasket bad or rivets loose & manifold sucking air.
You have two choices if mainfold sucking air.
1-Buy a new manifold.
2-If you can pinpoint area of leak try sealing area with red rtv gasket sealer or drill out rivets,clean out joint & seal with rtv sealer & reassemble manifold useing screws,lockwashers & nuts.
Onan eng are prone to this problem as they get older.