The reason your vintage version works so well is because of design. When they copied it for the new edition motors, they didn't maintain the same distance and angle. The vintage motor used an AX26 whereas the new edition motors used a cheap 4L290 FHP belt but is 1" longer than the original and changes over to an AX27. Even replacing the 4L290 with the AX27 often won't work, because the clutch lever doesn't have enough PULL!!!!!. The vintage clutch handle has a lot of PULL and moves the clutch arm enough to engage and dis-engage the primary belt.
Although I can adjust the clutch to make them work OK, never can I achieve the same level as the vintage version. Remember the vintage Whizzer clutch handle had a lot more PULL, and could loosen and tighten the front belt completely, whereas it is impossible to do the same with the new edition motors.
The reason the automatic clutch is so popular is mainly because the manual in next to impossible to adjust correctly.
You could replace the rear belt with a 1" smaller version, and that would cure the loose rear belt, but still won't fix the short range of the front belt tension. If you get the front adjusted to allow you to stop without killing the motor, then it will slip at high speeds. If you make it tight enough to connect at high speeds, you won't be able to stop without killing the motor [not enough clutch lever PULL!!!!].
If you have the time, and are convinced to make it work, then try using a 15 series belt. The 15 series belt is thinner and will release easier at the stop sign ,and can still pull at high speeds. The smaller 15 series will help compensate for the short clutch handle pull.
I would suggest trying a 15280 [15/32" X 28"], however belt companies vary greatly and it may need a shorter version #15275 [15/32" X 27.5"]. The thinner belt will need to be shorter than the original because it sits lower in the pulleys. Your best chance of making it work well enough to ride is by trying the thinner series belt or buy an automatic clutch.
Have fun,
