I have a misfire which varies in severeness. Pre-HE flathead

Also at about 4500 - 5000 rpm the tacho drops to zero, picking
up again once revs drop to about 4000.

It has a ReOpus ignition amp.

I use NGK Iridium plugs, previously I have run these with the
engine being turbine smooth.

Anything special to check before I start going systematically
through it all? In particular the tacho dropping indicates to
me I have a general problem possibly in the low voltage
circuit?

 

I agree.

The low volt side is high on my list, as is the coil, where the tacho feed comes from.

 

Thanks Grant. Should have mentioned I have tried another (used) coil so I will bring forward the job of replacing the ignition wiring. I am halfway making a new injection harness from hight temp wiring and high temp protective mesh tubing around the wires.

 

Besides the mesh tubing I also covered mine in this




I bought them on ebay
blue split wire loom | eBay

 

If you have replaced the coil I would suspect the ignition module as well as the harness. Those older transistor ignition modules had no overload protection.