I'm getting in way above my pay grade here
but the old Haynes book of lies
doesn't include the speedo sensor it says
"All models are fitted with a fully transistorised electronic ignition system, which due to its lack of mechanical parts is totally maintenance-free. The system comprises a rotor, pick-up coil, ignition control unit and ignition HT coils (refer to the wiring diagrams at the end of Chapter 9 for details). All models are fitted with two HT coils.
A throttle position sensor provides information for the ignition control unit. The ignition triggers, which are on the alternator rotor on the left-hand end of the crankshaft, magnetically operate the pick-up coil as the crankshaft rotates. The pick-up coil sends a signal to the ignition control unit which then supplies the ignition HT coils with the power necessary to produce" Chapter 5.1
Then further on it says
"The throttle position sensor (IPS) is mounted on the outside of the right-hand carburettor and is keyed to the throttle shaft. The sensor provides the ignition control unit with information on
throttle position and rate of opening or closing." 5.7
Maybe the ignitor is using the rate of change of the tps rather than the speedo or tacho signal?