Back in the late sixties I used to own a Bantam. It was only a few years old. This D7 was stock when I bought it for £40 and ran quite sweetly up to 60mph, then shook and shuddered its way up to 65mph. Of course, as a young hoodlum with friends on high revving Jap twins this just wasn't enough power and speed. I needed more but didn't have the bread to pay for it. Thus began a sorry tale of home tuning on an unwilling and ill equipped BSA two stroke single.
My first act was to remove the head and cylinder, applying file to the ports. I was able to find a used racing piston with gummed up rings an afternoon's cleaning sorted that out. Deleting the head gasket further increased the compression and radically advancing the timing completed the picture. The worn primary chain and clunky gearbox were left stock.
The new motor refused to start. It did spit flames out of the carb a couple of times (the air filter had also been junked). I changed the timing to a slightly more moderate setting. A lot of misfiring in the exhaust was eventually followed by a loud bang and an idling engine, immediately followed by large clouds of blue smoke out of the exhaust.
Out on the road the gentle power band was replaced by no power for the first three quarters of the throttle, then lots of spluttering that eventually turned into a burst of great speed, blurred vision and tremendous heat from the engine. Playing with the timing reduced these adverse effects a little and dismantling the baffles in the silencer returned some of the lost power.
I finally had a bike able to reach between 75 and 80mph, depending on road conditions, and one that with some abuse of the clutch could put the Nip invaders in their proper place. Handling was limited by the front forks that were designed for ageing commuters. As the SLS brake was as much use as praying to god, both were junked in favour of a front end off an A10. This made the handling rather heavy but as the Bantam only weighed 270lbs this wasn't much of a problem.
Unfortunately, the Bantam engine didn't take too kindly to being tuned and thrashed. The clutch was first to give up. Unused to so much power and to being abused because of the lack of power at low revs, it actually exploded. I spent an afternoon picking up pieces of metal out of the chaincase, an occupation that also revealed several large chunks of alloy missing out of the chaincase due to the thrashing primary chain. With a new clutch and primary chain installed, second and third gear decided not to work. The wild revs needed to change up from first to fourth resulted in some. ominous knocking noises from the motor. The crankshaft was actually bent. The bores were also scored from a broken ring that had sagged on the enlarged ports.
I picked up a used engine for £25 and restricted my tuning to smoothing out the ports and a high compression piston. The new bike lacked the zip of the other one, but it was much more reliable and could buzz along at 70mph. My friends were thrashing Jap crap with no problems and I had to look for a bigger bike.
Johnny Malone