There are so many oils available today that it is close to rocket science to work everything out. Mineral only oils like they sell at garden centres are probably closest to what your Bantam ran on in its heyday but even they have changed due to environmental concerns. The days of taking five miles to find out what was producing that blue haze in front of you have gone. A modern two stroke with a separate oil reservoir and modern oil won't smoke very much. A Bantam with a modern oil at a there or thereabouts pre-mix shouldn't smoke too much either. I would suggest buying an off the shelf semi-synthetic oil from you local petrol station. Get a litre container of petrol and add 40ml of two-stroke for 25:1 ratio and get another litre of petrol and 20ml of oil for 50:1. I use the measuring cup from my daughter’s hayfever syrup (an old one!). Try and have as empty a fuel tank as possible and a cleaned up spark plug for each trial and start the bike up. I would use the 50:1 sample first and take it for a few laps of where you live. Check what comes out the back and the colour of the plug once it is fully warmed up. Too much oil and you’ll be smoking and fouling the plug, too little and the plug will be whitish and the bike will be running hot. Too much oil and the fuel mixture entering the engine will be lean and lacking power, too little oil and it won’t lubricate what it is supposed to and it may seize. Don’t let the bike idle down any long hills because it will lacking on the oil and would alter the result. Ideally a beautiful biscuit brown plug, like they show in every Haynes manual I’ve ever seen, is what you want! Never be tempted to use engine oil even as a get you home measure as I have seen holed pistons and seizures as a result. Good luck and it would be interesting to know your findings.
