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MGB busy tach on 1979 MGB at 75 mph

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Tacho picks up a pulse via the dizzy/coil. Would start there and look for loose or dirty connections.
You may find that the problem is in the dial.
 
Grantura_MKI said:
Tacho picks up a pulse via the dizzy/coil. Would start there and look for loose or dirty connections.
You may find that the problem is in the dial.
Click to expand...

At 75MPH you are probably spinning close to 4000 RPM (unless you have an OD transmission in which case more like 3500) - could you be experiencing a missfire which just happens too quick to notice but causes the tach to drop, then pick back up, with the momentum of the needle causing the wild swings?
 
I thought about misfire but the engine is pretty smooth. I need to check wires to make everything is clean and well connected. The service manual states there is a loop of wire in the tach. I wonder if I put a loop of wire around a magnet like I have on some of my power cords? Toroid choke?
 
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