Speedwell tacho wiring

[30hp all the way]

Hi

Need some advice on the wiring on a speedwell tacho
The car is a 1956 VW oval beetle
When i Rev the car the tach needle goes up very slowly (not with the engine revs) and does not come back down.
If I keep tapping the tach, the needle slowly starts to drop back down.
I have tried swapping the pulse wire round (the loop wires that go to the distributor and coil) and it then doesn't work at all.

The pulse wire (the White loop wire), one end goes to the negative side of the coil and the other end of the wire goes to the condensor (the end that used to go to the coil)
+12v goes to ignition
-12v goes to earth

Thanks for any info

[D-train]

positive to a positive (igintion fuse)

neg to neg on coil an it should work.
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