Troubleshooting a Tach Problem need Help

[JGross]

I’ve installed a tachometer I got from www.classicparts.com in my 65 C10 that running a small block V8 with a HEI distributor. The paperwork with the tach said it would work with the HEI system however the tach is operating very erratically, instead of it moving smoothly up and down the RPM scale when the engine is revved it will jump up to 1000 RPM then jump again up to about 1800 and will not go higher.

Here’s my question, has anyone installed this tach on a V8 with HEI and had it work? If so I know that I have a wiring problem or a bad tach.

Thanks