650/750

Hi Gang, I have a 79 650 C that I'd like to put a 750 crank, barrels and head on then drop in a set of 83 GPZ 750 cams. Is this doable and will the stock carbs be sufficient. I want to keep stock exhaust and air box. If I do elect to go with larger carbs will the boot from lets say a 76 KZ 900 fit the opening for the airbox?
Thank sin advance, Bob

Hey Bob, Welcome to KZRIder.

There are a bunch of people on here with both 700 and 810cc big bore kits which would probably have more complete answers for you, but off the cuff, I think you can do most of that.

What 750 are you looking to swap over from? I was looking at doing the 810cc bore a while ago, which requires just the jugs from a 750, head and crank remain stock. Cams from a 750 are a common upgrade, but not all will fit. Last time I checked 750E cams do, and yes some GPZ ones, but again, not every model.

If you are going to swap out the crank, barrels and head, why not just shoehorn in the entire 750 engine, and sell the 650 unit complete to help pay for it? Seems like it would be a lot easier than cobbling everything together, and the only really trick bit is fabricating some engine mounts, and maybe chain offset. I would think that swapping GPZ 750 cams into whatever 750 you wanted to use a a donor for the rest would be a fair bit easier and quicker. There are a handful of those that have been done around here as well.

Stock carbs would do the job, but 26mm would be better, though you might trade off a little midrange for top end, depending on how you set them up. I have a set of 26mm Mikunis waiting to go onto my stock(ish) '650 (more flow through the polished/ported heads/pods and 4-1.), and KZ 900 boots for the years that also had 26mm carbs also work on the 650 head. Only problem at the moment is that you only seem to be able to find LHS ones new. So,to two of them 'point' up, but that's just a cosmetic thing....

Cheers,

Jon

Several years back Reborn650 (do a search here) put 750 cylinders on a 650 and bored it out to 810. Used the existing 650 crank, cam chain, head and mikuni carbs. As far as I remember he had to shave 650 cases where they met the 750 cylinders. He may have tweaked the existing 650 carbs. Lot of great info if you can find that (long) thread.
EDIT
A quick search tells me there may have been many threads and posts

I met him one summer evening and spent a pleasant couple of hours in his garage chatting while he checked cam timing etc.One great looking bike

I should have a photo somewhere

So long as cross forum posting isn't too heavily frowned upon:

I read through all of this while I was considering a swap up to a 700//810. That was before i found something else frivolous to spend lots of money on. :laugh: To be fair, it was my Norton, which still doesn't run... ah well.

forums.kz650.info/index.php?topic=7051.0

forums.kz650.info/index.php?topic=4096.0

It's a couple threads by the same fellow documenting his build, and a general how-to on going big bore on a '650. He's an engineer tho, so it's pretty darn comprehensive. Probably not a bad place to start if you are considering the build.

Cheers,
Jon

Thanks for the input. I've been through several 650's as they are a great bike for commuting on. Small enough to juggle from one lane to another in stopped traffic and as we all know potent enough to let the back tire drift when necessary. But I still want a little more. I read the 750 crank has a 66 mm stroke as the 650 has a 62 mm and the barrels are the same, pls correct me if I am wrong. Years ago I sold my Kaws, 500's through 1000's to a friend and picked up a CBR 929. Chased horsepower around until I was broke then picked up a GSXR1000 that spent its life in the Superbike series. Now my son is commuting in the Bay Area and needs a bike and the 650 comes to mind. Although he's 6'2 so I'm concerned it may be to small but he races the KX450 so my biggest concern is not enough power on tap which leads to this thread. I'm a toolmaker and as far as the carbs go I had planed on opening them up 1mm to help keep some of the mid range.
Hope this helps solve some of the madness behind the conversion.

The stroke is the same for the 650 and 750, it's the bore that is larger on the 750 (738cc).

Nope like others have said keep the lower end on the 650 motor (same stroke) and use 1980-81 750 barrels....... crank case needs to be opened up slightly too for the barrels to fit...... GPZ cam are hotter but has a different base circle (1.110 vs 1.070) that I wish someone would explain in detail of what that en tails those to fit vs the 650/750 cam ...... you swap out the 650 sproket's on them and since they (GPZ cams) use a electronic tack it has no worm gear for the tack drive that the 650 uses also..... 29mm smoothbores use the 900 manifolds along with the 26/28s ..... .... the mounting holes need to be filed out wards slightly (guessing .020 maybe a little more?) in a oval shape for them to fit but isn't too bad (round file) and the ports line up right on the money.

Search for more is a ton of info on here and on goggle too.

Also, for what it's worth, I am also 6'2", and I haven't found my 650 too small.
There are times when I have given a lot of thought to strapping on some rearsets tho, just so could tuck in a little nicer on the highway. But all in all, (Rob could probably comment on that, as he has a set on his, I believe....) I imagine that would be a good fit for your son.

Since the informed consensus is clear that the 750 doesn't have a different stroke, I guess it doesn't make a lot of sense to bother with the crank, but hey, if you're going to tear the bottom apart to check up on everything, then why not, and report back to us all on fitment.

Last bit: If you find yourself doing a bunch of searching on the kz site, I have sometimes found the built in search function to be a bit lacking or slow. I use the google site search function (enter "site:kzrider.com" after whatever is it you are looking for and google will return results just from kzrider.) to be really useful, and more intuitive. You probably already know that, but just in case. The backlog of archives here is really big, and I find it save me a lot of time when looking for something in particular.

Cheers!

Jon

Small kz gang in a garage with the custom 810 that looks like a 650.




Colin / reborn650 (striped jacket) surprised many a bigger bike from time to time.
difficult to see in the pic but he did have carb work done and fitted velocity stacks. I think he may have used gpz 750 cams but not totally sure. Lots of info in the archives

Or you could just shoehorn in a kz1000 engine with a 1425 big block and a turbo.

Here's another picture of Colin's 650 C1:

I've been thinking about making my KZ650B1 into a 750 or even an 810, in fact I have a complete 2nd motor that I could build up allowing me to keep the original. I've never been able to find a report though on just how much more usable power even an 810 conversion will give, and how it rates on the "Butt Dyno". While the GPZ cams give extra power etc. you are pushing the power to higher revs and I have to say my 650 is pretty powerful already, but we always want more