A while ago I had a very similar issue to yours. I didn't check the ignition or attempt any adjustment to the carbs at the time, but instead made a mental note to investigate the problem before the next outing. A week later, with the bonnet removed, I started the engine and noticed the middle exhaust gasket blowing so removed manifold and found that the middle of the centre gasket was completely missing. A bit of fettling later and new gaskets fitted the engine ran really very smoothly. Out on the road the difference was remarkable, no lumpiness just smooth response through the gears. Just a thought.