Just checked out my (UK spec) 1978 1500.
The heater circuit seems to have little to do with the circulation of water
to the block or head and I can see no way that it should effect the cooling
effect to any individual cylinder.
First there is a cicuit from the thermostat housing at the front of the
cylinder head, through the heat exchange set up for the carbs (SUs) and back
to the water pump. This operates all the time, flow being controlled only
by the thermostat valve which will help with quicker warm up of the carb
inlet manifold in cold condtions.
The heater circuit is a sub-circuit off this, and has no direct connection
to the block.
There are two "T" connections to the inlet manifold heater pipework, both
near the rear of the engine, with a valve in this to close it off when no
heater is required. When ther valve is open the water supply must be shared
between the manifold and the heater core. The circuits are in parallel, not
in series.
Guy
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