750/650 swap

Hey everybody,
I've been stalking this forum for a while now, and I've learned a lot from it. But now it is time for me to properly introduce myself. Why? Because I'm back at my bike.

It's a 1979 kz65D. I rode it last season, and it developed some very low compression through the rings. I believe this may be due to the vacuum port for the petcock being disconnected.

Anyhow, I've got the bike torn apart for a bit of restoration, so I figured I'd address the issue. New rings arent cheap, but recently I scored an 81 kz750h2 motor with good compression for dirt cheap, but it has a busted cam chain tensioner. I know it will bolt in. Here's my question.

What are my options to get the bike going strong again? Can I repair the tensioner, drop the 750 in and use the 650 ignition? Or can I bolt the cylinder block and pistons into my 650 motor? Or will I need to go buy ignition for the 750? If so, what years of 750 ignition will work?

I will post up pics of my "restoration" so far when I get home.

Any help is appreciated.

I would try to use the 750-4 engine & electronic ignition. The 750 has a Hy-Vo cam chain & slipper type guides (the 650 has a roller cam chain w idler sprockets).

Tensioner. I guess you could use the 650's tensioner - the gasket is the same on both engines. If not, you could get a manual tensioner by Ape Racing from Z1Enterprises.com or a Krieger manual tensioner.

Ignition. The 750-4 has electronic ignition: there should be a pulsing coil assy & timing advancer under the right side engine cover cap. Assuming the SR has points ignition, you'll probably need to get different coils such as the green Dyna 3.0 ohm. And an igniter from a 750-4 or 81-83 650-CSR. Or you could make an igniter. Or get a Dyna S ignition system + Dyna 3.0 ohm coils.

Charging system. The 750-4 has a 3-phase charging system w the 3 yellow alternator wires. I don't know what the 650D has, but if you need a different regulator-rectifier, I think you can use one from a Ninja 500 up to at least 2007 (I put an 07 EX500 reg-rec on my 81 650-CSR which has the same charging system as the 750-4).

Carburetors. You can use the VM24s (rejetted). Or, if you get some different carbs from a 750-4 or 82-83 650-CSR, - you'll either have to replace the airbox or modify the duct openings & use 750 ducts. Also, the 750-4 carb holders are different than the 650D.

martin_csr wrote: I would try to use the 750-4 engine & electronic ignition. The 750 has a Hy-Vo cam chain & slipper type guides (the 650 has a roller cam chain w idler sprockets).

Tensioner. I guess you could use the 650's tensioner - the gasket is the same on both engines. If not, you could get a manual tensioner by Ape Racing from Z1Enterprises.com or a Krieger manual tensioner.

Ignition. The 750-4 has electronic ignition: there should be a pulsing coil assy & timing advancer under the right side engine cover cap. Assuming the SR has points ignition, you'll probably need to get different coils such as the green Dyna 3.0 ohm. And an igniter from a 750-4 or 81-83 650-CSR. Or you could make an igniter. Or get a Dyna S ignition system + Dyna 3.0 ohm coils.

Charging system. The 750-4 has a 3-phase charging system w the 3 yellow alternator wires. I don't know what the 650D has, but if you need a different regulator-rectifier, I think you can use one from a Ninja 500 up to at least 2007 (I put an 07 EX500 reg-rec on my 81 650-CSR which has the same charging system as the 750-4).

Carburetors. You can use the VM24s (rejetted). Or, if you get some different carbs from a 750-4 or 82-83 650-CSR, - you'll either have to replace the airbox or modify the duct openings. The 750-4 carb holders are different than the 650D.

+1


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Thanks for the awesome reply Martin! Much appreciated. I've now got a rec/reg, coils and ignitor from another 81 kz750 LTD.

When you talk about the carb ducts, you are talking about the airbox end, not the motor side correct? I will likely just throw pods on there to solve that issue. I have carbs from an 81 750 and vm24s off a 79 650, but I think I'll use the vm 24s.

In looking at the motor, I am going to need to extract a couple of exhaust studs and replace them. Wish I'd noticed that before time to get to it.

The carb holders connect the carbs to the cylinder head. The 650D's are different than the 750-4 --- the 650's openings are smaller for the VM24 carbs. Since you have both sets of carbs, you can compare the difference. I think they will swap, but you might have to ream out the 650 carb holder bolt holes a tad. If you haven't removed the carb holders to swap them yet, you probably ought to get some Kano Kroil penetrating lubricant. The stuff may help in removing the broken exhaust studs as well.

The airbox ducts connect the airbox to the carburetors. The 650D airbox is different than the 750-4 's- the inner holes on the 650 are smaller than the outer holes (all 4 holes are the same size on the 750 airbox & the ducts are different). Mechainical slide carbs are more suitable for pod filters than CV carbs. If you use the 750-4 carbs, you'll probably need to get a 750 airbox & ducts, as CV carburetors can be difficult to tune for the pods.