Finicky Little 850 with SU Carb

If I understand you correctly, you are saying two cylinders at one end of the engine are running weaker or richer than the two cylinders at the other end of the engine. I presume you identify rich or lean running from the spark plug condition? If you have the standard one-into-four inlet manifold, there is no way over the dimensions of this part that the gas flow will be measurably different from one end to the other. It will all be pretty much turbulent flow and that will ensure even distribution throughout (unless you've got some special gas-flowed manifold ??). So I think these symptoms are more likely to indicate an air-leak - and the fact that it gets worse when the engine is hot, expanded and any cracks, splits or gaps will be larger is consistent with this. Hopefully you've just got a gap somewhere in the inlet manifold, in the region of the weak-running cylinders. Worst case you've got a crack in the cylinder head. The gap in the manifold might be as simple as the gasket leaking. Or it might be a fracture in the casting. Obvious first and easiest step is to remove the manifold and check it out. Reseat it with a new gasket, and you might want to think about a film of Hylomar Blue just to be on the safe side. If this doesn't cure things, then the next step is to pressure test each cylinder, which will show if the head is faulty. If it is, then the most practical cure is a replacement head, unless again there is any sign of gasket leakage allowing air to be sucked straight into the cylinders. In this case a skim and new gasket should cure it.