Hey Steve. With my old '77Kz650 Custom, I did both of the big-bore engine projects you are considering.
Last year I put a Wiseco 700 kit into the 650 cylinders, with stock rejetted Mikuni 24's, ported 650 head, cams, pipe and filters. The bike ran pretty good, way better than stock. Part of this performance increase off the line might have been the 14 tooth drive sprocket which makes a big difference for acceleration, but takes away from the top speed. I am more about hammering it through the gears than high speed highway blasts.
For the 2006 riding season I decided to go with a bigger plant with a bit more scoot under the saddle, so over the winter I picked up 750 jugs and head from an '80 750/4. After researching on the Kzr archives and speaking with some other 650/750 racers I decided on the following set-up
810 Wiseco for the bored 750 jugs. I was convinced to stick with the 650 head because of the small combustion chamber. I ported and polished the 650 head which was milled 10 thou.
I put in Kz750 cams, Gpz springs, 750 intake valves, Dyna coils and ignition, Custom tuned Wired George Mikuni 28's, MAc pipe, 14 tooth sprocket. I went with velocity stacks instead of pod filters for the retro look. They give a way-too-cool throaty roar that comes outta these chrome trumpets when I crack the throttle wide open. I don't drive in the rain or on dirt roads so the skimpy filters on the stacks are sufficient. This bike was built for short little blasts in the city usually late in the evening when there is less traffic on the road.
I kept the stock '77's Hy-Vo chain, pulleys, adjusters and cam chain sprockets, switching them onto the Kz 750 cams. The only modifications I had to make to the 650 block were to open up the bottom end engine cases/bores to handle the 750 jugs. I also had to put in a different bolt from the front and rear of the 650 head to connet to the 750 jugs-neither of these mods were a big deal.
This old girl with the 810 combo is a screamer and will spin way past 11K on the tach while still pulling. It would take a pretty well tuned '70's litre-sized bike to outrun me up to 60 miles per hour. Maybe the driver has some input on the impressive performance:)
This package has worked very, very well for me.
If you have any questions send me a message.
Cheers-Colin Firth-Ontario Canada
1977 Kz650 Custom, 14,900 miles bought new by my brother in 1977. Miss July 2006 Kzrider Calendar. Two different paint schemes, stock Moon Dust Silver and Custom Inferno Red with Silver and Gold Stripes. Superbike bars, bead blasted rims, engine, sculpted seat to hold me in when I crank it open .
2007 Toyota FJ Cruiser
2006 Chevrolet Uplander
Restored, 1974 Kawasaki KS 125 Enduro with 7200 miles.