Bad compression

I have just rebuilt my 1973 z1, new piston rings, valves, valve seats done, and basically everithing that needed to be done. I think I put everithing back perfectly; So I did a compression check after I put 30miles on the bike and cylinder 1 had 130ps, 2 was 120psi, 3 was 115psi, 4 was 130psi. The book said that all pistons should be 130-160psi. The only thing I can think of that I did wrong is the valve shims, could this effect the compression??? Other than that I'm not sure what else could be the problem..

recheck the valves and retorque the head bolts,, ride it ,put some miles on it so it will get broke in ,, then check it

is the motor smoking? if it is not then go put some miles on it and allow the rings to break in to the sleeves properly. the motor can't be expected to be broken in after only 30 miles go for a long ride this weekend 4-500 miles at least check your oil levels after evry stop then come back home replace the oil and filter save the filter so you can cut it apart to look for metal pieces. then redo your compression you will find them to be better i'm sure. and if nothing else you will have had a nice ride and figured out any other problems you might have.

If you need to seat your rings:

I recommend city riding where you accelerate through the gears, and do NOT cruise at a steady RPM.

The other thing is to put it in second or third gear and open the throttle wide open and run it up to about 1/2 of redline RPM, then let the throttle off and decelerate on engine braking. You only need to do that maybe a few times on each ride, but it does help seat the rings.

The main thing is to ride around varying RPM and throttle opening and stay below about 1/2 redline RPM.

00. wrote:

I have just rebuilt my 1973 z1, new piston rings, valves, valve seats done, and basically everithing that needed to be done. I think I put everithing back perfectly; So I did a compression check after I put 30miles on the bike and cylinder 1 had 130ps, 2 was 120psi, 3 was 115psi, 4 was 130psi. The book said that all pistons should be 130-160psi. The only thing I can think of that I did wrong is the valve shims, could this effect the compression??? Other than that I'm not sure what else could be the problem..

Did you hold the throttle wide open as you cranked for the compression test?

DITTO on the retorque of all head bolts, they have to be redone after the first run in.

Should allow engine to cool completely (preferably overnight, imo) before retorquing head.

Good Luck!

I got it fixed yesterday. The problem was all my shims were too tight. I din't quite understand how to shim this motor so when I did it the first time I did it like on my suzuki gs motor; but the z1 has a very weird special procedure to shim the valves. most car and suzuki motors are the same, you shim the valves when the cam lobe is up. But the z1 is shimmed in pairs at certain points. Now my compression is 145psi on three cylinders and 140 on number 2 cylinder. I'm happy with that. Rides like a dream now...

Ride it like you stole it.....:laugh: