Jet size in relation to elevation

An air (vacuum) leak downstream of the carburetor can most certainly change your air/fuel mix. A leak allows an excess of air to enter the equation after it has been mixed in the carburetor. It leans out the mix. A leak from a bad gasket can also change as the engine temperature changes. It might be worse when the engine is cold or as the engine heats. Either way, you need to assure that there are no leaks in the intake circuit. That means where the carb attaches to the manifold as well as the manifold to the cylinder.

Intake gaskets are easy and simple to make. Auto parts stores can provide good quality gasket material. A pair of scissors and an Exacto knife are the only tools you'll need although a gasket punch can be a nice addition to your tool collection.

An exhaust manifold gasket leak won't effect the running near as much as a bad intake gasket. It needs to be replaced, but it isn't as critical for a good running engine.

Tom

If your intake manifold is steel there's a good chance it is warped. The heat from the welding process does that. It might need to be milled, or sanded flat to get a good gasket sealing surface.

Here's a hint. Clean the flange then 'paint' it with a magic marker or machinist dye then run it over a sheet of sandpaper laying on a known flat surface such as glass. You'll quickly see any high or low spots.
The mating gasket area on the cylinder is usually in pretty good shape but you might want to check it the same way for any irregularities.

The same for the exhaust flange. They're usually warped too.

Tom