if there's an old dried out shrunken or cracked gasket between the carb and engine, air will get sucked in, making idle near impossible.
Also, this extra air intake can lean out the mixture, possibly overheat the engine causing mild seizure, loss of power and hard restarting.
Easy way to check is to just look closely.. if a gasket's condition is questionable make a new one. If a nut or bolt's threads are questionable, fix it.
You might try spraying some propane (unlit torch of course) around the intake manifold area with the engine running at a low, steady speed.. listen closely for a change in RPM, which would indicate propane is getting sucked into the leak.
I always use a coat of Permatex 2 (non-hardening gasket sealant) on the intake gaskets and metal surfaces to insure that there are no leaks .. the stuff can cure a gasket leak that is not too bad.
check all the bolts and nuts.. head bolts, etc.
It's possible the vacuum leak can be at the base of the cylinder too.. and since the top-end was disassembled recently it's not so unlikely.