You /5 has the Bing slide carbs, right? I'm not familiar with the slide carbs as well as the later CV carbs, but there are probably several things you can do to richen the carbs. The two main things that meter the fuel are the needle/jet (mid-throttle metering) and the main jet (3/4 and wide open throttle metering). As for the needle/jet, you could change the jet to a bigger size (this will be a small jump in richness) or you could pull the needle up one notch (this will be a large jump in richness). You could also increase the size of the main jet, but that only applies to heavy throttle usage situations.
Another thing you should look into is the settings for the floats. They also control how much fuel can be sucked into the venturi. Be sure the floats are set correctly. If the floats are too "low", then the float bowl will have little fuel in it and this contributes to running lean. If too "high", this is a rich condition.
If it were me, I would check with Bing or your dealer and get new everything in the carbs, especially floats, the needle and its jet...probably the main jet is OK. You should find out what the probably sizes are anyway. Put your carb back to stock specs with new parts. Go through a carb synching. Then, see what your plugs look like.
It would be best to start with stock settings and go from there.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2