Z1OwnersClub GB

Hi hope someone can help please. I have a 1976 A4 which when I got it it was running rough. I have done a top end rebuild on it, the head gasket had gone. Its all back together but I have this problem.
I have a good battery only a year old, fully charged reads 12.8 volts. When I connect it up it will start the bike ok and if I run it up to temp it will restart 3 or 4 times no problem.

Now if I disconnect the battery overnight to make sure there is no drawdown of charge then reconnect it next day it will turn the engine over noticeably slower and it fires but it will not start it. If I try again within a couple of revolutions of the engine it is as though the battery is almost flat. When I measure it it says 12.6 volts.

I have taken the starter motor out checked it cleaned it brushes, commutator etc and it seems all good.
I am not sure where to go from here, help!

Battery's can read plenty of volts but if it's not sending the amps required not much will happen.
Jump the bike off a non running car battery. If that then spins the motor over then your bike battery is dead.
If it's a gel battery that's been allowed to go near flat after charging they can show plenty of volts but deliver bugger all amps.

If the battery earth to the engine is corroded it can make the whole thing sketchy check lead where it bolts on to the motor.

OK thanks for your thoughts