possible leak?

There is dark colored dirt on the engine side of the rubber flanges that mount the carbs to the engine/head. Is this normal or a sign of some sort of leak?

I had the same thing. I recently just rebuilt my carbs from kits from Z1. I cleaned everything up and I suspected a leak from the dirt surrounding all those carb holders. What I did was unbolt them and put a thin layer of the high temp gasket maker (black RTV) silicone on the engine side of the carb holders when I remounted them after I cleaned everything up. So far so good.

Were you having a driveability issue beforehand or was it just a target of opportunity?