79 KZ400 Engine Rebuild Suggestions

Hey guys. After owning my KZ400 for 3 years now, I finally conducted my first compression test on it yesterday. I have bene avoiding it because I wasn't sure I WANTED to know what it was going to tell me.

Unfortunately, the results were not ideal. Both cylinders came in at 70psi. This actually came as a shock to me because (IMO) the bike pulls really hard. Of course if this has been the case since I bought it, I have nothing to compare it to.

Anyways. That being said, I am considering a (light?) engine rebuild in the next year. Wondering what all I should replace.

Things I know for sure:
1) Rings
2) Gaskets

Anything else?

valve seals while your in there

CHeck the valve guides also. Good luck with your rebuild!

How many miles?

You might not need rings if the mileage is less than 40k.

Definitely need a valve job, get some VITON valve seals while doing it.

I bought a new set of guides for my 750 twin because it had 80k miles, but the ones in it had zero wear when they checked them. Still have the new set if anybody needs them....

Thanks for the all input guys. I believe it is around 40k miles. But not sure if that is accurate or not.

Perhaps adjusting the valve clearances would increase the compression.
And would also re-test with a different compression gauge.

And if the low compression persists, would perform a leak-down test to determine where the compression is being lost (before engine dis-assembly).


A leak down test injects air pressure through the spark plug hole to pressurize the combustion chamber.

Can perform a poor man's leak down test by introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber. This may be done at TDC or any other cam positions where both valves are fully closed. Top dead center (TDC) is when the piston is at its highest position. An easy way to tell TDC is by alignment of the T mark with the case mark when viewed through the timing window.

May introduce a spurt of compressed air by using a rubber cone-tip blow gun (rubber air nozzle) held into the spark plug hole. And of course keep holding it in position to keep air from coming back out the spark plug hole while listening for escaping air at other places. The air compressor should not be running while listening because the noise will likely drown out any sound of escaping air.

Air heard escaping from exhaust port indicates exhaust valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from carb intake indicates intake valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from crankcase breather indicates loss of compression past rings into crankcase (perhaps worn piston rings or cylinders).

Air heard escaping from head gasket area indicates indicates loss of compression past head gasket (perhaps due to a blown head gasket).

A leaking valve may sometimes be resolved by adjusting the clearance to within specs.

A leaking head gasket may sometimes be resolved by torquing the head fasteners when the engine is stone cold (such as after sitting overnight).

Good Fortune!

Patton wrote: Perhaps adjusting the valve clearances would increase the compression.
And would also re-test with a different compression gauge.

And if the low compression persists, would perform a leak-down test to determine where the compression is being lost (before engine dis-assembly).


A leak down test injects air pressure through the spark plug hole to pressurize the combustion chamber.

Can perform a poor man's leak down test by introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber. This may be done at TDC or any other cam positions where both valves are fully closed. Top dead center (TDC) is when the piston is at its highest position. An easy way to tell TDC is by alignment of the T mark with the case mark when viewed through the timing window.

May introduce a spurt of compressed air by using a rubber cone-tip blow gun (rubber air nozzle) held into the spark plug hole. And of course keep holding it in position to keep air from coming back out the spark plug hole while listening for escaping air at other places. The air compressor should not be running while listening because the noise will likely drown out any sound of escaping air.

Air heard escaping from exhaust port indicates exhaust valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from carb intake indicates intake valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from crankcase breather indicates loss of compression past rings into crankcase (perhaps worn piston rings or cylinders).

Air heard escaping from head gasket area indicates indicates loss of compression past head gasket (perhaps due to a blown head gasket).

A leaking valve may sometimes be resolved by adjusting the clearance to within specs.

A leaking head gasket may sometimes be resolved by torquing the head fasteners when the engine is stone cold (such as after sitting overnight).

Good Fortune!

+1

And if you do need a rebuild might as well take her to 443cc