Model A Zenith carburetors are VERY simple and lots of info is available . I feel that if a person has a model A , they really need to know how to work on and repair their own Zenith carburetor .
Usually , the gaskets and the needle valve assembly is all that I need to replace as long as the jets are original .
Renners offers flow tested jets .
A lot of the repro jets are too short and will continue to leak , no matter what you try .
Look up original jet length and measure the jets that are now in your carb .
Original jets are numbered .
If your jets are too short and are below the level of the gas in the bowl of the carburetor , it will leak .
Advertised float level is 5/8s .
I lower the float level a VERY SMALL amount less than 5/8s , so as to reduce the possibility of the gas sloshing in the bowl of the carburetor when coming to a stop .
When the float level is really high , sloshing gas in the carburetor will cause the engine to quit running when a quick stop is made .
I don't EVER drill out the passage plugs because of the risk of ruining the housing of the cast iron carburetor .
Practice makes perfect . Good luck , whatever the choice !!!