KZ900 brand new rebuild, low compression

To get only 5 lbs of compression. A valve must be stuck open. Even no ring would give you higher than 5 lbs. I sure would not run it....Russ

cuddies say yee 925 wrote:

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.

The complete list of reasons for low compression:

1) valves not sealing: incorrect timing or clearance, bent, not fitted to seat, etc.

2) Blowing by rings.

3) leaking at head gasket

3) cracked head or block allowing compression path to escape.

I'm not going to buy it's running on four cylinders. Check your gauge, something is wack here. Let a Z get down to 90 psi, and see how it runs. It's not running on 5 or 35 psi! Whats coming out of the exhaust pipe? Better yet, what color are the plugs?

kzz1p wrote:

I'm not going to buy it's running on four cylinders. Check your gauge, something is wack here. Let a Z get down to 90 psi, and see how it runs. It's not running on 5 or 35 psi! Whats coming out of the exhaust pipe? Better yet, what color are the plugs?

The funny thing is, is there is no odd odors or colors or smoke coming from the pipe, looks completely normal. The plugs also all look the same, a little black tho.

cuddies say yee 925 wrote:

kzz1p wrote:

I'm not going to buy it's running on four cylinders. Check your gauge, something is wack here. Let a Z get down to 90 psi, and see how it runs. It's not running on 5 or 35 psi! Whats coming out of the exhaust pipe? Better yet, what color are the plugs?

The funny thing is, is there is no odd odors or colors or smoke coming from the pipe, looks completely normal. The plugs also all look the same, a little black tho.

Okay, have you tried another gauge on it? Make sure yours is right or you be chasing your own tail. Are all four head pipes hot? Quickly touch them when it is running.(did I say quickly, remember that word) Do they feel like they are the same temp.? Are the plugs wet at all? You said black, black wet or black dry? Then I'm going to ask if you have sync the carbs? If so, did it pull mercy up the tubes?
You could try just checking each cylinder with a plain old vaccum gauge at the manifolds, what is the diff per cylinder?
After you have tried the easy stuff, then go for the leak down tester. Most of all, remove yourself from the project for a few minutes. Stand back and look, and think for awhile, the problem could be right under your nose....

if two holes where dead like your gauge is telling you they are if you could even get the engine to run at all would smoke like the freakin exon valdez was dumping straight crude into it . the engine not running properly at lower rpms says to me a carb problem plugged pilot circuit or pilot jets. trust me when i say this from 18+ years as a professional motorcycle mechanic get a different compression gauge and try it again and make sure it is totally seated with no air escaping past the seal.

Man, I had a LOT of problems on this back in the day!
I'm the original poster, 12 years later... Lol!

The story behind the bike and the low mileage, and sitting since the 80's is all still very real. She sits at 6,617 miles as of now. I stopped working on this back in 2010ish, and it's sat for another 11 years. Here I am again trying to not only get it running, but fully road worthy, because my dad really is getting older now!

So, the bike is running now, but still hasn't seen the street. I started again on this project in March of this year. I sure wish I took all the good advice that was given to me here back in 2009, because I had a huge list of problems that I found this time around. I was a little snotty, for sure! I must say though, that 12 years ago there wasn't as much internet presence and knowledge around, I had an empty wallet, no owners manual, and thought I knew everything, but knew nothing. Here's what I found upon tear down, and what it took to get it running well:

- 2 of 4 cylinders had rings installed upside down.
- all valve clearances were out, 5 of them tight .000
- all 4 exhaust valves were bent. Intakes were fine. Replaced all 8 valves anyway.
- 1 coil was bad, or seemed so intermittedly. Both coils got replaced
- points gaps were arcing, gaps measured 25+ thou
- cam timing was 2 (or was it 3?) teeth off. Probably explained the bent valves


She now fires right up and seems to run well on the stand. Couple small things to work out, but that's normal. Once I finish going through it and getting it road worthy, I'm sure I'll have some fine tuning to take care of.

Thanks for joining along my ride.
I'll be around!

Welcome back. Looking forward to seeing this one on the road.

Found this as I look to address the compression issue on my 79 KZ650.
Thanks for your follow up.

Never give up!
Never Surrender!