What Randall said. Except that video doesn't sell home what is arguably the most important point to make in replacing this assembly in the carb.
I just finally got my first motorized bike build to work tonight. It took a while to get the thing started, for various reasons, but I was wondering why I could only adjust speed by adjusting the choke. The throttle did nothing.
If you think about it, this problem implies that you are already getting maximum fuel and air, and are simply modifying the air:fuel ratio by opening the choke to let in more air.
I realized there is a small notch in the throttle barrel. This notch must be aligned with the idle screw. If it is not, the idle screw simply prevents the entire throttle barrel from sitting down flush within the carb.
That is, as Randall said, rotate your throttle barrel until you see that notch line up with the idle screw. It actually helps to turn in the screw to prevent it from seating in unless it is in the proper orientation. Make sure it sits down flush, tighten everything back up, and have a go.
If you had the same problem as me, this will fix your issue, but create the new one of not having a good balance. You'll need to let out the idle screw and adjust the nuts at the top of the throttle barrel and at the throttle handle itself, until you have the appropriate amount of slack/tension so that the bike idles well.
What might help is to put a fair amount of tension on the throttle cable, and to open the idle screw most of the way (counter clockwise). Then get the bike going, engage the clutch, and adjust the idle screw back in until it idles at a good rhythm.
Hooe this helps! A lot of blood, sweat, and, well, cursing, went into figuring this out for my first time!
