The higher the elevation will be the lower you want the cyl. head. you need to lower the head because the engine does not have a dense enough air charge to compress. The lower the more dense the air is and the higher the head clearance will need to be. As far as the plug goes I have seen everything used from o.s plugs to nova. The hottest I would say would be a 5 and that would really have to be for freezing cold weather. I would not really see anything lower then a 6 in the engine. It is a .21 and also it is running 30%. You can also tell you have a hot plug by the way the engine looks on the top of the head and the piston. They should be a silky black with some carbon build up no detonation at all. You can start to detonate the engine with to hot of a plug that means that the plug will still be good but the combustion chamber as well as the top of the head are pitted. If the engine is doing that and the plug is still good you can be sure that the plug is hotter then it needs to be. With the relationship to fuel and humidity, Remember the basic fact that the rain or for that matter a humid air carries water. The engine likes and also does not like that. It will affect the needle settings greatly. There is only so much space inside the chamber for the engine to compress the air/fuel. So with that in mind the carb venturi opens the same amount all the time. And the restrictor you have just slows the air speed does not really limit the volume. This has been misunderstood for along time. The restrictor slows air. But you have to remember that the venturi diam. is still the same and you are only talking at the most a difference of what 2mm? From a 7 to a 9mm. Now that is understood. The engine draws in the same amount of volume of air filled with either less or more water from the humidity. There is still only so much space, now taken up by water particals. No room for more less air intake so I will let you answer less or more fuel is now needed? Let me know!!!
