Originally Posted by
jacksbike
I would try to find a 5 speed freewheel and a basic or as high quality replacement chain as possible. I believe that the 7/8/9 speed chains are narrower and have more chamfering of each link so that the chain slides onto the freewheel tooth more quickly and easily. I just remember that these Winner ultra 6 freewheels were weird in their spacing and tempermental without the correct chain. I think with the correct freewheel and chain that your problems should be easily solved.
Spacing shouldn't be the issue, as Ultra 6 spacing is identical to that of 7 speed SIS stuff. I recently made the change from an Ultra 6 to a 7 speed freewheel for use with the same 7 speed sis shifters and installed a KMC 9 speed Z chain on it, and the result was VERY nice. Although, I'm having to take the new freewheel back for a service, as it seems that the Pawls are sticking already after only 2 months or so...bummer. But as for shifting, the pair-up for me was beautiful. Hope you can get to the bottom of this.