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I've recently got my kz650 back on the road after 30 years in bits. The setup is 650 frame and gearbox, gpz 750 barrels and head, kz900 mikuni carbs and boyer electronic ignition. I'm hoping the bhp is at least 80. On top of that I've reduced the weight to just under 190kg (416lb) and I'm quite light. Having not ridden it for such a long time, I'm a bit unfamiliar with it but I'm sure it must be undergeared!! Any recommendations? Adding teeth to front sprocket?or lowering at rear sprocket? Or both?

16-42 is stock. Went with a 16-41 for years and liked it ( but not much difference). Tried a 17 tooth front and it killed outta the hole perf but with your added power it might be close. Am curious to see what others are running. I've always been a 1/4 mile guy and 15/41 was always my fav set up but will admit it revs a little to much on the road.

Have you ridden it yet? I like to get baseline first using the final drive ratio and RPMs at say, 65 MPH and see where you are at. If you are winding the engine up too high at 65 then you are geared too low. Alternately if you are slow off the line then you are geared too high. Put it in a spreadsheet and play with the # gear teeth keeping in mind you can only add so many teeth to the front.