KZ900 brand new rebuild, low compression

Bike is a 1976 Kz900 A4. Bike had 6000 miles, and sat since 88. Motor was just rebuilt, everything minus bottom end. Carbs were rebuilt as well. Has maybe 30 minutes run time on motor.

Compression is as follows..I am very dissapointed and stumped at the same time. Starting from cyl. 1 on the left, it blows... 5psi, 35psi, 120psi, 120psi. I have not torn it down again, but I cannot figure out why the two let cylinders are so low. I have checked valve clearances and thats about it. Could be bent valves, burned piston (but not much run time, doubt this)....and I don't know what else.

Any ideas to look for would be appreciated. Thanks, Erik

If the bikes been sitting for 11 years then I would say the rings in 1 and 2 are probably stuck. Pull the plugs and pour in about a teaspoon of marvel mystery oil in each cylinder. Let it sit a couple of days and re-test compression. If it comes up then try and start the bike. I have had the same problem with bikes that have sat for a awhile and always managed to get them started this way. Once the bike starts and the rings will work themselves free and the compression will usually come back to normal.

Bike sat for 20 years and then was freshly rebuilt all rings are new, motor has 3o minutes of run time.

A leak-down test should indicate where the compression is escaping.

Good Luck!

in twenty years of sitting a lot of things could happen it could have gotten some moisture from condensation maybe gas has leaked from the carbs onto the pistons as the bike leans on the left side and evaporated and gummed up the rings who knows twenty years is a long time why was it parked in the first place that reason may hide the answer. it is like saying it ran good before i parked it twenty years ago but forgot to drain the gas out of it so every thing is messed up.

THE BIKE HASNT SAT FOR 20 YEARS WITH NOTHIN DONE DO IT!

I've said it 3 times now, it was REBUILT AFTER SITTING FOR 20 YEARS. The motor is FRESH and has about 30 minutes run time on it.


I do not have a leak down tester.

check valve clearances,hopefully just valves hang open a little.

I've check clearances and all are decent. I could use a few new shims, but even then a small valve opening would not cause a compression reading of 0, unless the valve wide open- which in this case is not. Cam timing and points are set correctly.

i don't care how many times you said it the way i read it it said that it had been rebuilt twenty years ago you did not state that it had been done just a short time ago you are looking for answers to your problem and we attempted to answer the guestion based on the problem that was described, has the engine been dynoed or run hard during the breakin period does it smoke or make any weird noises does it even run all you said was that you did a compression test why did you feel the need to do that was there a problem if so let us know and we maybe able to shed some light on your problem, remember you asked for help not the other way around

I did not mean to sound rude, I was simply saying.


My father bought the bike back in '78, his buddy had put 2k miles on it, or whatever it may be. My father rode the bike from 78-88, when in 88 my brother was born and my mother forced him to stop riding. He [obviously] kept the bike, but let it sit without touching it. Well, 20 years later I put my hands on it to make a smile come to my fathers face. Me and my buddy rebuilt the whole motor, minus the bottom end, the cylinder head and carbs were done as well. It has had some hard start issues, I've been having trouble tuning it, gettting the carbs synced, so forth and so on. I decided it give it a compression test, because it seems low to begin with when using the kickstarter. Well, then it leads me to the situation I am at now, no compression on almost 2 left cylinders, and 120 on 3 and 4. Valves are all within loose spec, but still within spec to say the least. There is only about 30 min run time 99% of it is idle, so I have not run it hard enough or hot enough to burn up a valve or piston..as far as I know.

The way I look at it is, is I will have to tear it down either way, but I am just looking for some possible start points before I get into it.

Am thinking the engine would hardly run at all on just two cylinders.

As known, compression may escape through either valve, or the spark plug hole, or past the piston and compression rings, or around the head gasket.

Besides bent valve or holed piston, other possibilites might include:
Weak or broken valve springs;
Warped cyl head;
Loose spark plugs;
Damaged head gasket or installed upside down;
Broken or damaged o-ring;
Excessive ring gap or failure to stagger gaps;
Broken piston rings during reassembly.

Some auto-parts stores such as Auto-Zone loan tools to customers, and perhaps a leak-down tester would be available. Could ask around. And maybe also consider having a local motorcycle repair shop do a leak-down test.

Good Luck!

Here are some pictures to go with it all. Before and after.










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