Damn NGK numbers...from what I can find (NGK sent me two power sports catalogue instead of an automotive catalog...I should be getting the automotive catalog in a week or two) your plug crosses to a Champion RN11YC. If so, that may be a bit cold unless you are running 10.5:1 or more compression with a pretty big cam. I go at least a range hotter AFTER you get your carb situation straightened up. I'd also look for a plug that has a 5/8 hex rather than that 13/16 hex. The smaller hex is much nicer to use.
So you may want to see what a Champion RC14YC crosses to in an NGK and use that.
BTW, a platinum plug with an older ignition isn't always a good thing. They are harder to fire and if you are marginal on ignition that plug may be causing misfires because the ignition wasn't designed for that plug. You may not feel the missing, but it can happen.
A copper core plug is cheaper and much easier to fire. Yes, they require replacing at closer intervals, but you may be causing issues with a platinum plug you don't need.