Most gasoline engines that we use on our planes use a 5-7 heat range plug. These are medium/cool heat plugs. Watch out running the 2's and the 4's because they are a hotter plug and I have heard that you can burn a hole in the top of the piston after extended use. As far as "unloading the prop" goes going downline or in a dive and having poor performance issues I would take another look at how I re-assembled the carb...sounds like fuel to me. Exploded views are available for our Walbro carbs at www.ereplacements.com
Good luck, I am just trying to help you.