Hall sensor is within the distributor. It`s the gizmo that fires the engine at the correct time (ie, takes over what the points used to do)
When it goes down, you`ll chase your tail - I did anyway. :o)
I stripped and cleaned the carb
I replaced the coil.
Got so frustrated I nearly set light to the effing thing for the insurance money. :o)
Check:
theres a connector *right underneath* the distributor body, close to the rear of the head/block. Make sure it`s solid, and that the part that goes into the distributor body is sound and isn`t earthing out. The plastic is brittle and heavy hands can break it, allowing the three thin wires to the sensor to chaff and earth out.
Do a search in the FAQs for the Hall sensor test, which basically involves using a paper clip to establish that theres a spark everywhere EXCEPT the hall sensor, ie, checks the coil and wiring in 30 seconds flat.
IMHO, after years of piddling around with cars, the coil VERY SELDOM breaks down.
Good luck.

Sinbad