My 79 Darmah has recently developed a major backfire out of he vertical cyl. It starts great, then quarter mile down the road she backfires hard and sometimes loses the vertical all together. Then it comes back after some major backfires and runs well. But ALWAYS back fires hard on decell. It has popped so hard that the carb has almost popped off. New pistons, carb slides, rebuilt carbs....the list goes on. All work done in the last 1k miles and issue developed within the last 100 miles...... On the upside, my GTS is as reliable as a brick!