China tach doesnt work

"I ain't there", obviously, but I'd think this is a trigger issue. First thing is, I would ramp the v/ cm setting WAY up towards the higher voltage end. There is a high voltage spike in that wave form and you may be over-driving the input. Then play with sync/ trigger. You may do better using external sync/ trigger if that scope has one.

Are you using a 10:1 probe?

Those are a fairly slow waveform, so once you get the trigger/ sync right, almost any scope should work. The old ignition scopes were often electromagnetic deflection--like a TV CRT, so the bandwidth was VERY low frequency.

I have only had my own digi scope (an now old Tektronix TDS-220) for a few years, and have never used it for ignition. Previously, everything I had was analog. Old Tek 475 which now has capacitor and other problems