My reasoning for going up "one step" in heat range was because if there are un-burnt gases then, in my thinking, a hotter plug might give a more complete burn in the cylinder and not in the exhaust pipes. Bosch nomenclature is the higher the number the hotter the plug, went from a 5 to a 6 non-resistor plug. The opposite is true with NGK, higher number, colder plug. I've attached a web site for spark plug manufacturers, models and their heat ranges.
Thanks for the feed back
Wobbly wrote: There is plenty of un-burned fuel inside the exhaust. On decel, air gets sucked in through cracks and the fuel burns inside the pipes.
That's not correct thinking. There is always unburned fuel in the exhaust, that's why modern vehicles have catalytic converters to help burn it. On decel there's even more because air is moving through the carb (picking up fuel) and then through the engine where it is not burned.
Additionally, to get the flow dynamics correct, both valves are open at the same time. Air in the air filter rushes through the carb and into the engine because the hot exhaust is at a lower pressure. A lot of that rushing air contains fuel that's going right into the exhaust. Next time you adjust the valves, run the engine to TDC. One cylinder will have both rocker arms depressed. The rocker arms can't be tight unless the valves are open.
I've had assembled engines on the bench that at TDC you could see straight through the combustion chamber.... the valves had that much lift.