TOPIC:

So, I had the intentions of rebuilding my KZP, but with school taking up much of my time, and having a 76 KZ900 to work on, as well as two 78 KZ1000s, I really do not see the point in continuing on with the build.

Since I have my sights on one of my 78 KZ1000s, I am considering using what I can from the abandoned project, or selling what I can to help fund the 78

With that said, I had the

frame and all parts powdercoated,

cylinder head built by Cavanaugh (larger exhaust valves put in, new guides, valve job, etc),
APE K410 cams,

Cylinders bored for the new Wiseco 1075 pistons

I also have a new set of of wiseco 73mm pistons with bored out cylinder for the earlier engines.

So my question is, since the J head will technically fit the earlier engines, is it worth it to keep the cylinder head for my 78? Or just sell what I have and start fresh?
I know the J style cylinder head will drop the compression, but I am not sure if the 73mm pistons I have would raise the compression enough to make it worthwhile. Im also curious if the K410 cams would be a feasible option with that combination. I would definitely have to get some new carbs if the J cylinder head would work.

Thanks guys!

The J head would be worth keeping with all that work done but the compression will still be down 2 points to around 8.25 to 1 no matter what bore size as the pistons comp ratio is configured to the smaller volume of the early KZ heads.
For your info the early head is 36cc and the later J and GPZ are 40cc so that 4cc difference equates to the 2 point compression drop.
On 1015cc engine each 1mm of the bore = 3.84cc and on a 1105cc = 4.1cc so a head skim (or combination of barrel decking and head skim etc ) of 1mm will get the compression back up close to the original 10.25 - 1 of the 73mm Wiseco piston.
There is enough meat to take more than 1mm off the head and still clear the valve seats as I have done this myself but with the larger valve and 410 cams I would check the piston to valve clearance first although there should be no problems as the J valves sit higher in the head and is why J valves have shorter stems.

zed1015 is right about the compression drop!!! I am in the same situation of a 8,25 to 1 compression scenario with my 903 ccm cyl. /Piston bottom end. IMO i am wasting 20 HP at the moment with this Setup but a 1100UT engine swap is next on my to do list.
So if you swap to your J head you need a CCIA to use your APE K410 cams.



I am runing GPZ110UT cams in my J head now with APE sprockets for the Z1 cam chain.

Thanks guys!!

So with all that in mind, do I use the earlier cam chain and guides ?

Also, any recommendations for carburetors? All I can think of at the moment is RS34s. Of course, I have a set of BS34s, but I imagine an airbox scenario isnt going to be super practical vs a mechanical slide set with pods

Lordzacharious wrote:
So with all that in mind, do I use the earlier cam chain and guides ?

Yes, that´s the reason you need the CCIA (roller chain) on HYVO chain J/GPZ Modell heads.

Hi guy's, iv'e just been reading thru this older post and i have been trying to get some idea's or answer's to my problem.
I have a 78 KZ1000 bottom end with wiseco KZ big block barrels (1261cc 10.25:1 KZ pistons) and a 81 GPZ head. I recently pulled the eng down, and with a mechanic mate, we measured the comp ratio, it was a bit over 10:1, i keep hearing it should be 2 points lower? And to top it off, i had exh valves lightly touching pistons, cam lift is 431.
Any ideas,
thanks, Chilly.

Chilly wrote: Hi guy's, iv'e just been reading thru this older post and i have been trying to get some idea's or answer's to my problem.
I have a 78 KZ1000 bottom end with wiseco KZ big block barrels (1261cc 10.25:1 KZ pistons) and a 81 GPZ head. I recently pulled the eng down, and with a mechanic mate, we measured the comp ratio, it was a bit over 10:1, i keep hearing it should be 2 points lower? And to top it off, i had exh valves lightly touching pistons, cam lift is 431.
Any ideas,
thanks, Chilly.

Head or block may have been skimmed to correct the compression drop of the GPZ head.
GPZ head is 40cc and stock KZ head is 36cc.
1mm of bore is around 4cc give or take depending on bore size.
Valve contact is anyone's guess given all the possibilities but could simply be down to wrong cam timing if it's static (usually a tooth out) .

Thanks for the reply zed1015,
I didn't know anything about the CR drop when the eng was built around 20 years ago (only had the internet for 10 years). The block might have had skim to tidy-up after the sleeves were fitted, first GPZ head was stuffed, so this one was put on about 10 years ago, the guy who did the head work said it didn't need a surface, but i don't know how much machining it had prior to me getting it. So this is why it's got me stumped.
But i don't need to make a fuss over this, i'm happy with the comp, just puzzled as to why it isn't around 8:1. Hopefully i'm not going to want to put some boost into it.
As for the valves, if the timing is right, my mate is happy to fly-cut the pistons, or i suppose i could retard the exh cam a bit.

If you've been running this motor for 20 years the valves may just have been kissing the carbon due to over revving or weak valve springs.
Obviously the clearance and timing needs checking and any issue sorting.