Another Solex thread

Hi have just bought a mk1..(jetta) obviously the booted version of a Golf.
I have the old 1.3 engine with a Solex carb on top. Now Im not sure if this is a fuelling problem with gunk in the tank, or an air leak?
I drove it back from Teignmouth to Essex yesterday and after many miles and on the M4 it started to play up. Pulling the chock out cured it.
Would I be right in thinking its an air leak??