What is the effect of diagnostics mode on the pcm

[Dylan]

Might be worth trying a new coul. As the revs rise the bpw increases possibly spark breaking down?

[pman92]

Definitely sounds spark related.
Particularly as diagnostic mode is only really locking the timing and you recently adjusted the timing from 5 to 10 degrees.

Jayme might be onto something. Wind it over to 10 degrees BTDC and take the cap off. The rotor button should be pointing virtually directly at the electrode in the cap going to #1 lead. If the rotor hasn't come around far enough and is before the electrode in the cap it's possible the larger gap it has to jump when it has more than 10 degrees timing could cause a misfire or even fire the wrong cylinder.
If that's the case it's an easy fix. Remove the distributer and refit it in the correct position and ensure the leads are in the correct position to ensure rotor alignment.
This could also potentially explain your uncle noticing it's firing when the timing marks are no where to be seen. Maybe that spark was intended for the next cylinder in the firing order?

If that's not the case, why did it previously have 5 degrees of timing? It doesn't generally move.
Has the balancer spun? Take #1 plug out and confirm TDC

[pman92]

Also stress the coil and see if it keeps firing without breaking down.
Put a screwdriver or something in the end of a lead, and sit it or tape it so there's at least 20 or 25mm gap between the screwdriver and ground.
Should see a consistent blue coloured spark jumping the 25mm gap