I have a 1970-1972 7.5 hp Mercury outboard, not sure of the year s/n 3802784. It idles and runs wide open fine but stumbles from idle to roughly half throttle. Carb is clean and fuel is new. It has the Thunderbolt ignition and I suspect either the trigger or coil but not sure. It acts like the timing is off as it pings when it is stumbling. Any advise??? Hate to just throw parts at it. Compression is 130 psi in both cylinders.