swapping for a newer engine

Two of us share the shop, my Nephew (GPzInfested) and me, and GPzInfested has a 79 KZ650 SR that he put a 83 GPz750 motor in, and 83 GPz750 front and rear suspension (so much for my GPz750 project bike). He also has a 77 or 78 KZ650B with GSXR front and rear suspension that we were going to put a 1100 motor in (but the deal for the donor bike seems to haven fallen through), but are now planning on a KZ/GPz 750 motor. My 80 KZ750E has GSX front suspension and wheels, and I'm in the process of putting a GPz750 swingarm on it (got to find me a TIG welder cheap). It's getting a 810 fitted KZ750 motor with MegaSquirt EFI (got most/all the parts, just a matter of time).
GPzInfested's out terrorizing the town on his GPz750 Turbo at the moment

And I have a couple of 750 twins

Steell, sounds like you guys have already done what I'd like to start doing - combining old and new and making your own style of bike. What the cafe racers were about in another time...

Can you throw me some advice on what else besides the motor I'd have to replace if I'm going to
a) bore out my 650 to a 750
b) get a 750 head, block, cams, pistons, rings
c) just swap in a 750 motor without opening it up

Thanks.

saszta wrote:

Can you throw me some advice on what else besides the motor I'd have to replace if I'm going to
a) bore out my 650 to a 750
b) get a 750 head, block, cams, pistons, rings
c) just swap in a 750 motor without opening it up

Thanks.

I'm not real clear as to what you're asking.

Are you asking whats required for each of the three option?

Or are you asking what other mods you should do to the bike in addition to each one of the three options?

Yup, I was asking what's required for each of the options. First time doing this...

Also, If I pick up a 750 block and head then if the 750 is basically a 650 bored out I should be able to use everything from my 650 (coils, ignition, plugs, exhaust system)
Just need to rejet the carbs without having to replace them?

Thanks.

Colin (Reborn650) makes a case for using the 650 head on the 750 blocks for a 750/810 conversion on a 650 bottom end, and I'm not going to argue with him since his obviously works. But I believe his 650 head is ported and uses 750 intake, or 750 intake and exhaust valves, I can't remember which.

750 pistons and cylinder block should give you a 12-15% increase in power simply due to the increased displacement and compression using stock KZ650 cams/head/carbs/etc, KZ750 cams will give you a couple of hp more.

I am running the stock 1977, 650 bottom end as these engines are pretty much bullet-proof. I fitted 1980 Kz750 cylinders bored to handle the Wiseco 810 pistons, 1977 Kz650 head, milled 15 thou, ported and polished plenums (on the exhaust side only) GPz750 cams and intake valves. The exhaust valves are stock 650's as they are the biggest that will fit if using the larger GPz intake valves.

Nice little package that really loves the higher rpm:)

Cheers-Colin Firth-Ontario Canada

1977 Kz650-810 with Wired George Prepped Mikuni 29 Smoothbores and lottsa go-fast goodies tucked away inside.

1982 Ferarri 308 GTSi, Rosso Corsa Red, Tan Interior, 3.0 Litre V8, 5 Speed. Mint Condition with only 51K original miles

2007 Toyota FJ Cruiser- Yellow with a six speed