I have 2 clutch pulleys, one with a small outside diameter of 3 1/2" and the other has a small outside diameter of 4", they both have a large 5" outside diameter. Which is correct for a vintage Whizzer? Thanks
The 3 1/2" is more common.
Since Whizzer's don't have a transmission, The 3 1/2" will give you the balance you need for good take off and top end speeds.