Hi there. Does this happen particularly after a fast run, and the engine is really hot? If so it could be a build up of carbon in the combustion space, holding the heat, and causing the fuel to ignite even without a spark from the plugs. Of course a de-coke would be best, but a quick fix that might work is to spray some carb cleaner through the carb, revving the engine at the same time. It should shift some of the carbon. In the old days, Redex was used via the spark plug holes, so I guess their petrol additive might work too.