What spark plugs for 340

Scody,
No I haven't used the brands listed in post 34. But I have tried Pulstar plugs & got a noticeably smoother idle with them.
I am also grateful to Charles Kettering. The story goes: his friend was starting his car with a crank handle when the engine kicked back & the handle broke his friend's leg; friend subsequently died from infection [ early 1900s ]. Kettering was very upset & vowed to find a better/safer way to start an engine....& invented the starter motor.
My earlier point was/is that we should be thankful some people took risks to improve a product or device.....or we would still be using crank handles.