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Andrew, I don't know where you are or anything about your engine, but the Standard ones over here nearly all do it. There are several reasons for this: The downhill pops and bangs are normal for a hot engine with a carburettor. What happens is that you run 'weaker' on overrun going down hill so the mixture does not burn completely on the spark alone - eventually you get an explosive mixture in the manifold/exhaust which is then ignited by the hot gases just leaving the cylinder. At least it lets people know you're coming, and it gives nervous horses and riders a chance to move out of the way before you get there. The missing and irregular beat idling is most likely some wear in the distributor - again, we are talking 50+ years old here, with no electronic fuel injection stuff (YUCK) so its not unreasonable. A new distributor would probably fix it/make it less bad on tickover/idling. Pops and bangs when cruising sounds like a slightly weak mixture, or that you have an air leak in or near the carb. The carb/manifold gasket and the carb throttle spindle are both common causes for this. Hope this helps.
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