Hi, as you can guess, I run a Ner-a-car, and have quite a bit of experience on them, particularly the 2-strokes. The largest of these is 285cc, with a measurement of 70mm bore by 74mm stroke. In the original English handbooks it gave NO timing figures at all! But the US handbooks did! Even then it was very vague. It depended on the fuel that was available.
I set my engines, which have fixed flywheel ignition, at about 5/32" BTDC. The US book suggests a max of 5/16" for the worst fuel, retarding it to between 3/16" and 1/16" for the better fuels.
It sounds as if you may have a manual magneto fitted. If so, I would suggest setting it at TDC fully retarded and then try it, then by adjusting the timing by the lever you could find out which setting it goes best at. I would think that you'd get a reasonable range of adjustment using the lever.
Incidentally, a friend has just bought an Omega. I have yet to see it - it's apparently a 1921 model, and looking at Tragatchs book may have an engine of about 275cc.
