I just fitted an Ignitor to a new replacement Lucas distributor without vac. advance which came with a basketcase SV. The tach worked on cold start but would cut out after a short run. I fitted capacitors which gave me a minimum reading of 2500RPM at idle no matter what I did with the adjustment screw. I finally fitted the needle at the "M" of RPM and moved it in a preload fashion to rest against a rigged stop pin at 0 RPM. That is the only way I could get things somwhat straightened out. I get a credible reading at low RPM’s after what the instrument’s reading is irrelevant.
I had to modify the Ignitor’s mounting base to lower the pickup coil because it sat well above the magnetic rotor and gave no spark at all otherwise.