This is moving on from your original ‘challenge’ to identify what’s happening to Roger’s excellent comments re timing (and efr’s memory test of high school tri functions – I passed, but it took about 10 min to work out why it was right).
Re timing of the more modern machines (mine is a ’59 Brum).
I have a copy of the Book of the Scott noted as ‘latest edition’ but without a date and an address of 2 St Mary’s Row Birmingham which includes recommendations for the 1949 model and onwards. This notes that the distributor includes automatic advance and gives ’17 – 20 deg at 2,900 rpm’ and the setting instruction is to have the right hand piston at TDC and rotate the distributor ‘until contact breaker points are just fully open’ and, obviously, the rotor connected to the right hand HT lead. This ensures ‘the spark occurs slightly before TDC when fully retarded in order to give the correct advance at at higher engine speeds’. It also notes that the auto advance commenses at 300 – 350 rpm.
I also have the Miller wiring diagram and instructions for a 1959 model (they are actually written for a Douglas Dragonfly, but this has been hand crossed out and Scott written in). This came directly from Miller when I got a new distributor cap in 1967. This states that the ‘the unit is fitted with an advance and retard mechanism giving 25 deg of advance at 2,500 rpm’.
So, the message to me is that when setting up the timing of the more modern machines, it is necessary to include the correct allowance for the auto advance.