Backfires are when fuel travels through the cylinders without combusting then explodes in the exhaust pipe. Fuel supply/combustion issue. It seems likely there's a problem either with the mix going into the cylinders (carbs fuel lines or air intake), or with timing. These can affect a bike at particular speeds and throttle positions. If using the choke has helped at some point that's a further indication (the choke changes the gas/air mix). ,,,
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