[Dyna S ignition Voltage drop 1977 GS750]

Hi

I have Dyna S ignition and Dyna 3 ohm coils

I have a  voltage drop (2v) going to coils and very weak spark

It still runs but cuts out at higher revs in 3rd 4th and 5th

I have checked the earths and fuse connections and kill and key switch  All OK

When I unplug Dyna S ignition I dont get a drop  in voltage

Any ideas??

thanks