How to measure an old motors RPM
It makes sense you are not getting any signal.  The magnetic field from the current in the spark plug wire is circular around the wire.  To pick that up, you need a wire wrapped circularly around some of those magnetic field lines.

What you want is a ferrite torus around the sparkplug wire, then the pickup wire wrapped around that.  Portable tachometer strobes use this principle.  The problem is getting the magnetic toroidal core around the existing wire.  This is usually done by having the core be only 3/4 or so of the way around the wire, with a movable separate ferrite piece that can be slid in place to complete the magnetic circuit once the wire has been positioned "inside" the partial torus.  The pickup wire is wound around the fixed part of the ferrite torus, with the movable ferrite hust helping complete the magnetic circuit to get more flux thru the fixed part, which gives you a better electrical signal.