Two real good bantam gurus are Alan in NC and Shane in Oz (I think I got their handles right) I played with D14's for a little while but the knowledge of these gents outstrips mine.
I was sure I'd posted a reply a couple of days ago, but here goes again.
Thanks for elevating me to the rank of guru, but I thought Alan and Allan were the Bantam men. And, of course, "Villiers" is Mr Two Stroke.
First up, fuel mix.
BSA quoted a 20:1 mix, using SAE 40 oil. Modern synthetic two-stroke oils can be mixed much leaner than that, partly because they lubricate better, and partly because they mix better and stay mixed. I'd be inclined to go with somewhere between 25:1 and 32:1. The reason for 32:1 is that it's 1/4 (Imperial) pint of oil to 1 (Imperial) gallon of petrol. The ratio used comes down to ease of calculation to a very large extent. A US gallon is smaller, so 1/4 pint to a gallon will still be in the general range. If the spark plug whiskers too often, you can lean out the mix.
Second, factory settings for the D3 AMAL
According to Roy Bacon
's BSA Singles book (yeah, I know, but the carb appendix seems to be mostly correct):
Main jet: 90
needle jet: .107
needle position: 3
Third, racing after start-up.
The mixture needs to be enriched considerably for a cold start, either by tickling or by closing the strangler.
Once that initial rich mixture has burned off, the mixture is leaning off excessively, which is causing the racing.
If it's only doing it with the throttle largely closed, it might be as simple as the mixture screw.
Otherwise, it's most likely to be an air leak somewhere in the system downstream of the carb. Early Bantams had a stub-mounted carb. These can suck air around the stub, so it might be worth removing it and refitting with a sealant such as Loctite 515.
The crankshaft seals tend to dry out over time and suck air into the crankcase, especially if the engine has been sitting. Replacing them involves at least a partial engine strip, so you don't want to go there until you've confirmed that everything else is alright.