Re: Spark Plug?
I use a BH7S in my 66cc 2stroke. Why? After reading the plugs, the 7 showed the best ground strap. I think the NGK website has a guide to select proper heat range. The ground strap should show discoloration about halfway to the base ring of the plug. Lower compression heads may need a hotter plug. Plug gap can also effect the heat range. Using a smaller gap may need a colder plug.
I used to use a projected plug, but if I didn't index it right, the extended ground strap would just barely hit the piston. I didn't feel it was worth it to keep the projected plug for this reason, and I didn't notice a performance gain from the projected plug.
Iridium and other such plugs are not going to give a performance benefit. They may last a bit longer, but is that really worth it? I find special more durable plugs most useful in engines where it's difficult to get to a plug, but it's so easy to replace these plugs that I'd rather just pay $1.75 for a standard plug. Electrodes are electrodes, some exotic material doesn't make the engine produce power out of nowhere.
A resistor spark plug can be useful if you have electrical interference. I never had problems with interference, so I use a standard plug. I have tried a resistor plug, and it made no difference for me as far as plug output. I also run things off the white wire. Maybe I have an exceptionally good coil?
These are my opinions. Some people may quietly nod and think I have good advice, some people may stand up on their chairs and throw their monitor out the window at my terrible advice. Either way, it's what I do, and I've had good results. These engines all run a bit differently, so it's worth it to play around with different combos.