The key word was "old". Your engine may be worn out. You also may want to install the stock carburetor and see how it run with that. Your problem may also be the carburetor you chose to use. The O.S. Max .40 FP did not originally come with a remote needle, so changing the carb may be the source of your difficulty, although the carb used on the .40 LA with remote needle should work nicely, too.

Since you didn't mention which carb you actually did put on the engine, we'd be able to offer no advice on it.

One thing to try is to mount the engine on a test fixture and run it there. That gets everything in the open and you can easily monitor the fuel system, prop, and engine at a comfortable working location.