It's an '84 Yamaha QT50 (Yamahopper), with about 4k miles on it. I don't really have a reason to suspect the oil pump, other than that I've already covered the most obvious bases... I'm running the plug that the book calls for, gapped within spec. I pulled the head, but not the cylinder. I guess that's my next step. I'm pretty sure there is no dirt in the carb (right after I've cleaned it thoroughly), and I now have an inline fuel filter in front of it.
The bike won't rev up like it should. I can't get enough revs to engage the clutch properly. It is topping out way below where it should, before it starts 4-cycling. The plug looks ok, I don't think I'm running too rich, but a smaller jet is definitely on my shopping list.
The way I figure it, since nothing I've done so far has made an obvious improvement, I haven't yet found the problem...