5) Throttle cables need to be replaced.
Just a small thing here,.. the B4 only has one throttle cable. It's not a push-pull like the bigger bandits.
I vote that it's likely cable routing. These little bikes really can be picky about the routing. For instance, route it wrong, and you'll find it that after a rev, the rpms will take a while to come down, or the engine will race or die if the cable is routed wrong (or pinched) and the bars are turned left or right. It's almost like the cable is 1/2" too short, so it has to be bang on, or you'll get that, even with a perfectly good cable.
That said,.. correct routing is in the service manual, that you can find in the Members section.
I find that the easiest way to test to see if the cable and the grip side of things is ok, is to leave it all wired up, and remove the tank (helps to alleviate any pinches or pressure points.) and rotate the throttle a few times with the bike off. roll it on, let go,... does it snap back? roll back slowly? Stay put? Stop prematurely? That'll give you a really good idea as to where to start.
Don't worry, you're not going to flood the engine for the next time you turn it on or anything, the juice only flows when there's vacuum pressure.