Welcome to the mind bong.
Fuels and fuel additives can influence the colour of the plug to a certain extent too. I found it really hard to get good plug readings using GRO.
There are a few other tricks you can pull out of your hat. If you do a WOT run under load for a bit and then pull the clutch and drop the throttle do the revs drop immediately? If not you're a bit lean. If you run WOT under load and then very slightly back the throttle off. Like just a smidge, does the bike feel like it surges forward or does it immediately slow. A small surge indicates lean. You can also do things like reading the fuel ring at the base of a new spark plug. That means sacrifing the plug (doing a single WOT run and then cutting the threads off).
At the end of the day you won't do too much damage being rich. You will nip an engine up if you run too lean. A rich main will splutter as you whack the throttle open in 3rd gear. A few sizes smaller will be the good spot. Running on the edge of perfection doesn't leave much room for error in the event that you get a cold morning or a load of deep sand.
I'd expect the 168 to be just about perfect for a 36mm carb, but you'd also expect the 39mm oval bore to want more fuel.
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