Engine Rebuild

Hey all, I'm thinking about freshening up my 77 650 this summer. I want to rebuild the engine because I think she is losing compression and I want a bit more power as well. So I'm gonna get a head job from ape, a carb job from wg, and I'm gonna get her honed and re ring it. What I'd like to know are any suggestions for what I should replace while I'm in there? I've got some idea but I'd like to get opinions on whats necessary and whats not.

In the Filebase under the KZ Information tab at the top of any page on KZR are pdf copies of a series of articles on rebuilding the KZ650 motor. Well worth reading.

yeah i haven't been around for a while, i'll have to have a look in there. thanks steell!

Get APE to open up the heads with a porting job and maybe get your hands on some Gpz 750 intake valves to have them install while they have the head. I don't think these services will be too expensive. APE might also mill the head slightly which will bump up compression. Send them the complete head and also get them to degree the cams. This work should get you some extra power.

When you are inside the motor check that the cylinder head is true. I am guessing that you will be having the cylinders honed by a machine shop. They can check the mating surface to the head. Give a once over to all of the idler gears as the bearings might be worn.

Why not pick yourself up a used set of 750 jugs, cams and pistons off ebay. A set of aftermarket rings aren't too expensive and the extra cc's and cams, along with the carbs and headwork will really wake up the motor.

Just a few thoughts as it is pretty easy to spend someone else's moolah:-)

Cheers-Colin Firth-Ontario Canada

Thats exaclty what i'm talking about colin. Ideas like that. That'd save some cash, and make some more hp! I would imagine those 750 parts would all add up to less than just a set of hotter cams from ape.

What year 750 jugs will work on my 77 650? Do you knwo off hand?


So a bit of background. I rebuilt a lot of my bike a couple years ago, dyna-s, alloys, chain and sprockets and so on, but i still feel somethings lacking. So i want to wake up/refresh my engine to get a more powerful, more reliable powerplant, and also do some suspension/brake upgrades while i'm at it, add some sticky tires, you know....hot stuff. I still dont know whether those gsxr 750 forks and AP Racing brakes sitting in the corner are going on her. They're for a different project, but thats on hold......

My cylinders are from a 1980 750. If I recall, they were simply 650 cylinders with larger liners pressed in by the factory to bring the displacement up to the 738 cc's. of the 750/4 designated size of the engine which was introduced in 1980. This way you can use your 650's roller cam chain and idler gear rollers without too much any hassle.

You will have to open up the bottom cases to handle the larger cylinders from the 750 but this isn't that big of a deal although you will have to pull the motor from the frame and split the top half of the cases from the bottom end. Sounds tough but it really isn't and can be done in an afternoon with a friend and a box of bubbly brew.

Even the stock Kz 750 cams will offer better performance than the stock 650 cams. A Fresh valve job, porting on the intake and exhaust runners and maybe even the Gpz 750 intake valves will let the motor breathe better. You will have to cut new intake valve seats to handle the larger face of the Gpz valves.

These are all upgrades that I did to my 650 head which has smaller combustion chambers than the 750 head and in theory, better bottom end power according to the research I found. BTW, I also added the 810 kit and Gpz cams which pretty much took this motor as far as I want to since it is no longer allowable to have a bottle fed street machine in Ontario.

Steell can chime in on some other items I might have missed as he has a pretty good memory when it comes to the parts interchangeability of the 650/750 motors. I'm still trying to blow out the winter cobwebs and thaw out my brain.

Keep an eye on ebay as cylinders, pistons, cams and Gpz valves often pop up for a pretty reasonable price.

As far as the lighter, more modern suspension I haven't gone that route with my old girl as I like the classic mag wheels and front end of the '77 Custom stocker.

Cheers-Colin Firth-Ontario Canada

Great info Colin. What do you mean about opening up the bottom end to fit the cylinders? A bit of machining? I plan on pulling the engine anyhow, the frame could use a fresh paint job, and a few hours with some spray bombs on the nice paint i put down a couple years ago should make her look good. I was a bit hasty with my colour choice then though, not many people like the blue frame and wheels, so I'm going to go with flat black from now on.

As for the suspension, Progressive shocks out back and springs in the forks will probably be the plan. I really like to look of my old bike, once everything gets a flat black paint job anyhow. :laugh:

The cases split in half on the horizontal, meaning you have a top half and a bottom half. The bottom half holds the crank, clutch, starter and transmission. The top half is simply a protective case that included the cylinder head studs and bore openings that the four cylinders fit into. There are probably about 8 or 10 long bolts that are needed to be removed to separate the cases.

The top case's inside bore for the 650 are slightly smaller than the 750's and therefore need to be slightly enlarged to handle the larger cylinder sleeves. I placed my top cases into my work bench vice and gently ground away some of the metal on the 650's bore. I used a variable speed hand drill with a couple of different shaped cutting bits, a rounded-end cutting bit and a more flat-ended round bit. These bits are pretty inexpensive and can be found at any local tool supplier.

Be very careful not to take out any more of the case's metal than needed and I would suggest taking off a little amount of metal then doing a test fitting, marking with a Sharpie where you need to continue trimming the bore opening.

I must've done twenty test fittings throughout the exercise which took a couple of hours (I go slow when it comes to working with metal as it is tough to put back on once removed!)

Once you have opened the cases enough, the larger 750 cylinders will snug in nicely.

Sounds tricky but it is pretty simple and will give you a chance to take a look inside the bottom end at the spinning guts for wear and tear.

Cheers-Colin Firth - Ontario Canada

Take a look at the bottom of this post:

kzrider.com/index.php?option=com_kunena&...98216&catid=2#198216

I just got done doing this to mine and there are some pics in there, although not everyone has a mill availible to do it on so you'll hafta get out the die grinder if that is the case.

the 750 set up is nice, for my money I would rather bore out the cyls to 1st over , if possible, then you would have new pistons nice clean bores and a head that is done right, if you are goingto go that far, might as well look at the bottom end and oil pump too, make sure everything is in spec

77_650B1 wrote:

Hey all, I'm thinking about freshening up my 77 650 this summer. I want to rebuild the engine because I think she is losing compression and I want a bit more power as well. So I'm gonna get a head job from ape, a carb job from wg, and I'm gonna get her honed and re ring it. What I'd like to know are any suggestions for what I should replace while I'm in there? I've got some idea but I'd like to get opinions on whats necessary and whats not.

I just did that on my KZ-750, rebuilt the engine from the piston rods up. I was initially going to just hone the cylinders and install oversized rings, but I elected to bore up one size and install new pistons and rings. Mine had 75k miles and definite cylinder wear. Yours may not have a lot of cylinder wear, you can see once it's apart.

One thing I learned: most cylinder compression blow by is not at the place where the rings meet the cylinder wall, it is in the piston grooves where the rings seat. Pistons are aluminum, so the walls of the grooves wear and loosen up which lets compression seep by. Even if you only hone, you may want to install new pistons and rings. I found some pretty cheap sets of NOS pistons and rings. The amount of work and pain is so much I wanted to make sure everything was tight inside.

The other warning: use best quality gaskets and avoid a nightmare. My aftermarket gaskets failed and I had to do the job over.

Also, I recommend getting new valve guide seals made of VITOn which is a trick new rubber that is much more durable than regular butyl rubber. Ebay has people selling VITON seals. The seals on my 750 failed pretty quickly after the last head job so I think it's worth it to upgrade those.

BSKZ650 wrote:

the 750 set up is nice, for my money I would rather bore out the cyls to 1st over , if possible, then you would have new pistons nice clean bores and a head that is done right, if you are goingto go that far, might as well look at the bottom end and oil pump too, make sure everything is in spec

That's also what I did. I had 75k miles on the engine so I just assumed the pump would be worn and I ordered the new internal gears and cylinder body to renew the pump. One of the old pros later told me that KZ oil pumps don't wear unless abused with dirty oil.

He was right: when I took the pump apart and measured the clearances, it showed no measurable wear. So, that was one thing I didn't need. YMMV.