The stationary speed of the engine in your classic car will not 'drop'. As a key man, you have already screwed in on everything, checked the time of the ignition and adjusted it accordingly, and it all did not help. Probably there is 'false' air somewhere between the carburetor and the intake manifold, or the intake manifold and the cylinder head. The assembly is therefore not airtight despite the installation of new gaskets. 'False' air means that it becomes much too hot in the combustion room and that can lead to damage. Burnt valve (s), a hole in a piston. Bad stuff. To detect any leaks, without having to dismantle everything again first, a tip: start the engine (make sure it is at operating temperature), then spray out the oil - which is best to have a spray can with a 'hose' - first the packing area between carburetor and intake manifold. Your 'assistant' will then see if (suddenly) white smoke comes out of the exhaust ... If that is not the case, then everything is not leaking. Then repeat that action in the area between intake manifold and cylinder head. If your companion still doesn't see any 'white' smoke, then it certainly won't leak there either. Well, then there is nothing else to do than dismantle it again. Maybe a gas valve does not close completely (due to wear?). Good luck. And strength! Classic driving remains a challenge.