What have I done wrong on re-assembly

The motor on my RS has been rebuilt (2 Lt Pinto).
Problem I have is after the motor has been running (at normal operating temp) it won't re-start. It struggles to turnover (almost like the battery is dying). Retard the timing and it will start.
Timing is set at 10 degrees which is what the spec was for the dizzy (H & H). It was no different when I was running the standard dizzy with points. I need to retard it from there to re-start (but that compensates the acceleration.
Should I be looking for a spark issue (weak earth) or is the problem likely to be somewhere else?
Motor is new and hasn't done much more then bed in the cam, so its still pretty tight.
Problem I have is after the motor has been running (at normal operating temp) it won't re-start. It struggles to turnover (almost like the battery is dying). Retard the timing and it will start.
Timing is set at 10 degrees which is what the spec was for the dizzy (H & H). It was no different when I was running the standard dizzy with points. I need to retard it from there to re-start (but that compensates the acceleration.
Should I be looking for a spark issue (weak earth) or is the problem likely to be somewhere else?
Motor is new and hasn't done much more then bed in the cam, so its still pretty tight.