Keith... thanks for the reply. Can you expand on what you said or provide some links to related material? It makes sense but I'd like to get a better understanding of it.
The depression in the carburettor ("suction" does not exist) venturi depends upon the velocity of the air flow through it (Bernoulli).
Suction does exist -
Suction - Wikipedia, the free encyclopedia
It's a bit more than just velocity -
Pressure