Sounds like a vaccume line has popped off,,,or you've developed a Leak in either the carb base plate gasket,,or intake gasket.
Have a good look see,,,and tighten and check all the hose connections,,,take a look at the Vaccume advance module on the Distributor,,make sure it's not leaking,,,and is advancing as it should.
If non of this corrects your problems,,,then get a small propane torch,,,and start your engine,,,then turn the propane torch on without being lit,,,then direct the gas to-wards the intake manifold to head seams,,,,same for your base plate on your carb,,,and if the RPM increases then you know you got leak because the propane gas will be sucked into the bad seal,,and will add fuel vapour to your engine which will increase the RPM.
P.S. also check the Vaccume host going too and from your brake booster,,,this one is the biggest vaccume lines you have.