I may be buying an old Nighthawk motorcycle this weekend that has a broken tach. Tachs on this bike can run 250+ on ebay. I could buy and aftermarket tachometer, but there is no fun in that!

What's the best way to actually pick up a signal from the spark plug wire?

I've seen other people do it with wraps of wire, but that honestly sounds unreliable and prone to problems.

Any other ideas on that one?

 

I don't understand what

What's the best way to actually pick up a signal from the spark plug wire?

means.

What are you trying to do with the spark?

 

I may be buying an old Nighthawk motorcycle this weekend that has a broken tach. Tachs on this bike can run 250+ on ebay. I could buy and aftermarket tachometer, but there is no fun in that!

What's the best way to actually pick up a signal from the spark plug wire?

I've seen other people do it with wraps of wire, but that honestly sounds unreliable and prone to problems.

Any other ideas on that one?

You pick up the signal on the low voltage side as a starter.

 

I've seen other people do it with wraps of wire, but that honestly sounds unreliable and prone to problems.

That approach (inductive pick-up) actually works just fine. The only negative is the nuisance of have a small wire wrapped around your sparkplug wire. I think the traditional automotive tach was connected to the ignition coil primary and detected a zero-crossing each time the points opened. So a tach had to matched to the cylinder count.

 

A hall effect sensor shrink-wrapped to the spark plug wire should do the trick.