kz750 twin pistons/rings

Btw incase I didnt mention it the compression doesnt really change . Cold or hot the compression is the same . Thats what stumps me

Does the engine have to be warm for the leak test to work? Ill test it tomorrow , will put cams and shims back in exactly how I took them out . lots of measurements and pictures

For reference, if needed --

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

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).

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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.

May alternatively use a portable compressed air tank.

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!

Schmeck wrote: what feeler guage size do i need ti measure my clearance ? smalled i have is .008 then .010 and neither of them fit . the lobes are up and the shim is touching.

For 750 twin: original manual spec was .002 - .004" for both intake and exhaust, but I believe the spec was opened up to .002 - .006". I keep mine .003 - .005".

Schmeck wrote: My valve shims that I pulled out had numbers on them. Intakes were both 260 , front left was 250 and front right was 245. After measuring these I can conclude that the front are still 250 and 245 but both rear shims measure 2.58 which is pretty close to 260. What is the gap that I should have between the shim and lobe? I don't understand why all the shims are just about what they say on them and all of them touch without a gap. Now from what my brain is telling me, If there is suppose to be a gap, and the shims are what they have written on them. That would mean who ever put these shims in did not put the correct size. I read somewhere that the gap should be between .010-.015 .If that's true then my clearances are waaaaay off.

Gap often shrinks over time. A neglected engine can have clearances go down below spec. You have to check them every 3000 miles. Gap is .002" - .006".

And the shims are 32mm, nobody makes new ones or sells them. Z1 enterprises used to have some sizes in used shims.

Schmeck wrote: I did a leak test and my compression didn't change, and from what Ive been told these seals go bad and leak oil onto the spark plug.

That's true. Mine has done it several times in the 80k years of service.

Schmeck wrote: My teacher in my college class whos a retired motorcycle mechanic told me that if My bike is at 90 for compression on both sides, that a faulty valve seal can cause that compression drop.

It did on mine but not directly due to leakage past the seal. The "oil track" buildup that oxidized onto the valve seat over time did gradually reduce compression and cause poor running because the valve would no longer seat.

Schmeck wrote: So I went ahead and bought 4 of those Viton valve seals that I hear are much better than stock ( made sure they were for my bike). Im pretty sure its these seals, but if it isn't then I can atleast cross off valves seals from my to-do list . :lol:

You will need the head overhauled and rebuilt by a competent shop but as somebody pointed out: you MUST have accurate valve clearance values corresponding to shim thickness. The shop will finish the valve end to set height but they have to know the height they need to get.

The 32mm shims are not available in many sizes and you would be wise to proceed carefully or you won't be able to get the shims you need to get the thing set up again.

The only shop near me that will even touch my bike ( because its a 79) wants 850.00 to go through the head. That is a lot of cash that I do not have. I have put 2 motors together myself though, So I think I might just take this out and fix it. After the cam chain is loosened/cams are out, what else do I need to do before pulling the head off? Want to leave pistons and cylinders in the motor so I only need the top off for the valves. If that makes sense,

Also do I need a special tool for loosening and tighting the cam chain bolt on the front of the motor. I use a cresent wrench which works well but is a pain .

Schmeck wrote: The only shop near me that will even touch my bike ( because its a 79) wants 850.00 to go through the head. That is a lot of cash that I do not have. I have put 2 motors together myself though, So I think I might just take this out and fix it. After the cam chain is loosened/cams are out, what else do I need to do before pulling the head off? Want to leave pistons and cylinders in the motor so I only need the top off for the valves. If that makes sense,

Do you have a machine machine shop close by to grind the valves and seats?


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Schmeck wrote: So I think I might just take this out and fix it. After the cam chain is loosened/cams are out, what else do I need to do before pulling the head off? ,

You need to get a factory service manual or you will have no hope of getting it right. There are three 10mm bolts upward also holding the head on.

Theres a couple of auto mechanic machine shops near me but many of them arent that great . Which is why i do alot of stuff myself . Also I have a factory manual just have to take the time to look at it . Once I take the head off I will try to post pictures for opinions because Many of you will look at the valves differently than myself . How does the whole socket trick work to get the valve keepers out? It involves a socket and a magnet from what i hear.

so even after removing the main and hidden bolts, Couldnt get the head off lol So looks like its staying on there for now. Ill replace the valve seals and see if there is any difference . Bike runs great just had the oil on plug issue so its probably the seal .