KZ650 to 750 Conversion - Cylinder Block & Pistons

Well, I got my 650/750 engine back together and all went very well. The 750 cylinder block and pistons came from a 1983 model. The lower part of the sleeves fit into the 1982 650 crankcase with no problem. Steell was right. Kept the 650 cylinder head and cams, but did some porting on the head and lapped the valves. The 750 piston pins fit the 650 rods perfectly. I measured the cc on the combustion chambers. Then placed a piston into the combustion chamber to the same depth that it runs in the engine. The 18.4 cc as compared to the 184.5 displacement (738 \ 4), should yield a 10.0 : 1 compression ratio. I suspect that the original spec. of 9.5 : 1 is over stated. The engine balance is still very good - no problem whatsoever. The fact that 2 pistons are rising at the same time 2 pistons are lowering, probably means that they offsset each other in the amount of weight being moved. The 750 pistons weighed about 7.9 oz. and the 650s weighed 7.2 oz.

I could feel a drastic difference in compression even when turning the engine over with a 17 mm wrench. The starter works harder to start the bike, but it starts quickly and really comes to life. Also, when I shut the ignition off it stops instantly. And then the way it runs - is a whole new bike. Great torque and power. I went one jet size richer than stock on the carbs. The motor responds to the slightest throttle movement. And all this was done for a total cost of about $115. I am extremely pleased with the results.

Special thanks to the TALLGUY. He shipped me a piston ring, since one of mine on one of the 750 pistons was broken. Saved me lots of grief. Thanks again Andy.

If you were to install the KZ750 (not GPz) cams I think you'd be even more pleased.
They drop right in.

Thanks Steell. Are their reasons to avoid the Gpz cams, other than their not having the tachometer drive gear on the exhaust cam? I do not want to have to go to an electronic tach arrangement, so that is reason enough, but I wondered if other problems arise.

It looks like the 1980 - 1983 750 cams carry the same part number except for the Spectre, Gpz and Gpz turbo.

Does anybody know what the lift is for the 750 cams? My 650 is only about .300".

Thanks to all who helped me out on this project.

hey, I think the 750 block is taller, you may need 750 connecting rods.

no, I checked the stroke is 54 on both.

donthaveakawman wrote: hey, I think the 750 block is taller, you may need 750 connecting rods.

Uh news flash,people have been doing that mod for 30yrs! :whistle:
Hes already got his back together

donthaveakawman wrote: hey, I think

There's the problem right there.
Maybe less thinking and more shut the F$$k up.

well this older generation has got another thing comin'!

First time I saw a 650 with a 750 top end was around around 80-81

why not just get a 750 and tell people you converted a 650 to a 750?

donthaveakawman wrote: well this older generation has got another thing comin'!

This older generation has managed to survive worse things! :woohoo:

:woohoo:

yeah, it's called another 4 year term until the next election.