1973 z1 Help on COMPRESSION

I have just finished rebuilding my 1973 z1 engine, new piston rings, all new gaskets, valve seals, basically the head is totally rebuilt. The bike runs fine, but when I check the compression; number 1,2,4 cylinder gets 180psi and #3 piston gets 150psi. But the book said that the compression should be 130-160psi, so why do I have 3 pistons that have 180psi? Also why would one cylinder have lower compression than the other three, can it be valves not adjusted properly? Or can it be some other problem. Please help....

Most likely that improperly shimmed valves are causing your problems. The valves being to tight can cause high compression so your #3 cylinder is probably just about right. Easiest way to check is to check the valve clearances and switch out shims as necessary

valves being to tight will cause lower compression, not higher.

00. wrote:

I have just finished rebuilding my 1973 z1 engine, new piston rings, all new gaskets, valve seals, basically the head is totally rebuilt. The bike runs fine, but when I check the compression; number 1,2,4 cylinder gets 180psi and #3 piston gets 150psi. But the book said that the compression should be 130-160psi, so why do I have 3 pistons that have 180psi? Also why would one cylinder have lower compression than the other three, can it be valves not adjusted properly? Or can it be some other problem. Please help....

Something is not right with #3. First check the valve clearance. If thats ok, redo the compression check then do #3 again but with some oil squirted into the cylinder. If the compression comes up there is something wrong with the rings. If it stays the same then a valve is leaking. Were the valves lapped?

since you just rebuilt it, check all the valves again to make sure they are right,, then ride it, I dont think you have let it fully break in.

So if the valve is not adjusted properly; or does not have the right shim it can leak?? I didn't know that an improper shim could make a valve leak, and will this leak make the compression lower???

you may be miss reading the ideas,,,

yes a valve too thight,, ie, no clearance and holding the valve open will "leak" since the valve is not allowed to close all the way.

yes a valve that is shimmed too tight, but still has clearance can make the compression number lower since it is holding the valve open longer and opening it sooner,, ie, letting more air escape prior to compression.

and the same can be held true if its not allowed to open fully and thus get the proper amount of air in the cyl.

its always a good idea to run it for a few hours, retorque the head bolts, and re check the valves, just good practice.

I still think its not fully broke in, meaniong the rings still have not fully seated

I agree with BSKZ650 on the break-in. How many miles have you put on it?

Does it run good?

Is it running on four cylinders?

Did you do a valve job, with your rebuild?

Alright I checked all the valves shims and only one shim was correct, all of the other shims were too tight I couldn't even fit a .038mm feeler gauge in any of the valves. So I changed all of them to the correct specifications, except on valve which I did not have the right shim for. I fired it up again and now my compression is 150psi on all 3 cylinders but number 2 cylinder is now 110psi. Is this correct?? Can I lower my compression by changin the valve shims??

put oil into all the cylinders and retest. Suspect that gauge, cuz what u said just aint right. not logical caption. cant figure it . have no idea. what have uuu donne?:woohoo: