What is the correct compression ratio for a KZ 900

Hello all,

I have conflicting info on the kz 900 compression ratio. My compression ratio is 112 -115 hot across all four cylinders. Is this OK ?
I have heard the Clymer manual says 130 and another manual says 115. Sure could use some help on this. Bike has 9500 miles is runs really bad. I would hate to spend $$$$ on a rebuild when not necessary.

Thanks,

Jerry

Jerry, I went and grabbed my Clymer since I relied on memory before. It says 130-160 psi. But remember, this is not a model/year specific manual. Covers Z & KZ900-1000cc chain & shaft 1973-1981 (Clymer M359-3). The other manual I got from here online and it the Kawasaki Shop Manual Z1 Series and seems to be from around 1972. It states 121 lb/sq in standard and 85 lb/sq in. service limit. Hope someone else with more information can chime in.

Did you pull all 4 plugs out and crank with the throttle wide open and with the battery fully charged?

sure did.

tnx taildragger

tnx Joe. When I added a teaspoon of oil to each cylinder compression jumped to the high 160 range

That is not a compression ratio,8 to 1,10 to 1 is.

jerry chabino wrote: Hello all,

I have conflicting info on the kz 900 compression ratio. My compression ratio is 112 -115 hot across all four cylinders. Is this OK ?
I have heard the Clymer manual says 130 and another manual says 115. Sure could use some help on this. Bike has 9500 miles is runs really bad. I would hate to spend $$$$ on a rebuild when not necessary.

Thanks,

Jerry

I wouldn't be happy with those compression #'s. There is a big difference between dry and wet for one thing, but 115, you would need to be around 8K feet above sea level. The bike would run and should be smooth, but less power with those #'s. So you may well have other issues. Ignition and carburetion come to mind. Possible vacuum leaks etc.

jerry chabino wrote: tnx Joe. When I added a teaspoon of oil to each cylinder compression jumped to the high 160 range

Sounds like its time for rings atleast if the bores are still in spec,but If Im going to all the trouble to pull the top end off Id go ahead and have a real valve job done including new seals if the guides are still good.
Dont forget to check cam chain and related stuff and atleast drop the pan and check oil pump pick up screen

The numbers you are talking about are cylinder pressure, not compression ratio.

Compression ratio is a mechanical number that won't change with engine wear, cam timing, oil in the plug hole. Cylinder pressure will.

decent rule of thumb is,
for any internal combustion engine.
it takes 65lbs of compression to even create combustion.
a good number is the 140-160 lb range with a compression tester in five kicks with the throttle open.
if you are using oil to help your numbers you are grabbing at straws.
100 lbs of compression is time for a rebuild.
anything less is dieing a slow death.

this info is free,take it for what its worth,your mileage may very,caveat emptor.

leon

I have had engines with low compression numbers that had been sitting for awhile. The rings get stuck and after getting it running then running it for a few hundred miles the compression numbers would come up to normal.

baldy110 wrote: I have had engines with low compression numbers that had been sitting for awhile. The rings get stuck and after getting it running then running it for a few hundred miles the compression numbers would come up to normal.

This can also be the case,just depends on the circumstances and how much time/patience you have.