low compression

kz900, 1976 model, bikes been setting for 10 year, the bike has 32,000 miles. It is 100% stock down to the airbox and the exhaust.

Low compression in all 4 barrels.

Seems to me might be one of 2 things, but its not holes in the piston tops, they are fine, I checked that with a flashlight.

Might be:

1) piston rings are stuck in the grooves, - OR -

2) valves are stuck open a bit, not fully closing due to surface rust/gunk on the stems from setting for 10 year

Whats the best way to fix it if the 'stuck piston rings theory' is right?

I do recall seeing some thread that said 'you don't need to pull the top end, theres another way to get the rings working again' something like pouring penetrating oil down the barrels or maybe it wasn't penetrating oil but another *appropriate* liquid, I don't remember that bit.

Then letting it set for a few days, kick it over some, then more of that liquid, repeat until the compression test improves.

What is the method and/or that liquid? I cant find that thread. I want to try that before I pull the top end.

I should have added, the compression is the same across all 4 bores.

Also, there is no head gasket leak, and no base gasket leak either, no holes in any part of the engine, and it turns over fine (using the kickstarter).

My suspicion is that, if the valves turn out to be the problem, I'd be surprised, because -- if the motor sat motionless for many years (10 years in this case) -- some of the valves would have been in the 'closed' position for all those years, and some would have been open, so if a 'stuck-open' valve was the cause of low compression in one cylinder (say, cylinders 1 and 4) the other cylinders (2 and 3) would be okay.

But if all four sets of piston rings have lost some springiness and/or are stuck in the ring grooves, that might explain why all 4 cylinder barrels are showing the same low compression.

So I'm really leaning towards the piston rings.

what compression gauge did you use?
did all cyls. read the same?
when kicking it over does it feel like it has compression?
how about valve lash?

How low exactly is the compression? Are you talking 70psi each or 110? Does the bike run? I dont know what you should do besides check valve clearance first off, supposedly seafoam in the crankcase can "unstick" rings... but I have never tried it. I have pretty low compression on my 650 - 115-130 across the board using a pretty cheap harbor-freight gauge that didnt seem to seal perfectly, but the bike was only ridden a few hundred miles in the 16 years prior to me getting it... Probably stuck rings or gunky valves as it only has 10,000mi on it - I would think likely the same issues on yours. I'm going to check valve clearances when I get a chance, as well as dump some seafoam in the crankcase but I'm also curious to see what other options there are.

Someone who knows a lot more than me will hopefully help out shortly!

Best of Luck

did you run the compression check with the throttle wide open? if not that could account for low numbers on all 4 cyls

hi mate its more than likely sticky rings ,any engine stood for a long period will loose compression as oil runs from the piston rings .dry bores & rings mean low comp.
get some redex or similar upper cylinder lube ,auto transmission fluid is a good second best . remove the plugs ,get the engine midstroke on any cylinder & bung a good spoonfull of oil in every cylinder .turn the engine over very slowly plugs out ,some oil will splurt out ,no probs.put some rags over the plug holes & leave standing .
next drain the engine oil as it will be past its best after being stood .
drain down the carbs ,you should really strip them as they could be full of gunk & cranking the engine over can suck residue into jets etc & making you more work.
when your ready new oil in the sump carbs filled with fresh fuel etc ,spin her over plugs out with rags over the holes to catch the oil residue .
fit the plugs & you should be off,there will be plenty of smoke but it should restore compression on sticky rings .

Thought I'd chime in since I have the same year engine on my LTD, almost the same miles (34k) and sat for 5 years. My compression was 90-70-60-30 and my leakdown test had air going everywhere. Might not have been too accurate since I could run the engine beforehand.

Anyways, I took the head and the jugs off and took them to a machinest. The cylinders weren't bad and the rings look ok (but I'm changing them anyways). The head was my main problem. One bent exhaust valve (#4) and all my exhaust valves were sticking in the guides and two had to be replaced. Should get everything back next week.

Hope that helps

Well folks, here's what I've found so far with my 'low compression' problem. Just fyi, I bought this '76 kz900 from the prior owner's family member and will not be able to ask the prior owner what kind of work he might have done on the bike.

The bike has been sitting for 27 years, not 10 years, and mostly indoors, with just a year or so of sitting outside here where I live in Silicon Valley, California.

I sprayed penetrating oil into the cylinders, kicked it through, sprayed in more penetrating oil and let it sit for a bit. Then I re-did the compression test, and by the way the throttle is cranked wide open for that (someone asked about it) and I checked the valve clearances, figuring that stuck-open valves can explain the low compression, and here's what I found:


Cyl. #1: Compression is 36psi. Intake and Exhaust valve clearances are below spec (the gap between the cam lobes and the shims is too small) at less than .038mm -- that means the valves are not seating. If the valves don't seat the compression might be found to be low -- and it *is* low. Could mean a new valve seat is needed, hard to believe though with less than 30,000 original miles on a 100% stock kz900. More likely the valve seats are just worn and the clearance has naturally reduced as you would expect. So I'm going to follow the FSM and install thinner shims to open up the intake and exhaust gaps to be within spec (0.05mm to 0.1mm) so that the valves will seat, then re-check the compression in #1.


Cyl. #2: Compression is 60psi. The intake valve clearance is just barely within spec at .102mm. The exhaust valve gap is WAY too big and out of spec, at 0.3mm. That means either (1) the exhaust valve stem's has some surface rust preventing the valve from closing so the gap reads too big + the valve is staying open which explains the low compression, or (and this is entirely possible), -- (2) the prior owner pulled the cams/top end at some point and did not keep track of which shim went on which valve, so all the shims might be in the wrong spot. Not far-fetched, I did this dumbshit move myself back in 1979 first time I pulled a kz1000 top end off, the shims come out when I was pulling the head off, I was trying to save time by not removing the buckets and shims after removing the camshafts and the head jerked free all of a sudden like, and all the shims popped out.:S


Cyl. #3: Compression is 120psi. But the valve clearances are both too big at .229mm (I), and .127mm (E). So I'm going to assume, with the decent compression reading, that the valves are seating. (Just how do you explain an increased valve clearance over time like this!!!)
I'm going to install thicker shims to reduce the gap to be within the spec (.05mm to .1mm) and re-check the compression. (HEY NOW, VALVE CLEARANCES GET *LESS* OVER TIME, NOT MORE! The clearances were set at the factory to be 0.05mm to 0.10mm, HOW DID THE CLEARANCES GROW TO .229mm and .127mm after 29,000 miles?!! WHAT IS UP WITH THAT!!)


Cyl. #4: Compression is 30psi. The Intake valve gap is too small, less than .038mm -- means the intake valve is not seating and installing a thinner shim there should allow the valve to seat and (hopefully) raise compression. The exhaust valve gap is WAY too big at .2mm meaning either there is crud on the valve stem preventing the valve from seating, or back to my theory of 'Prior owner pulled the head and all the shims popped out and got put back in the wrong place.'

I think I need to do a visual on those valves that have WAY TOO BIG of a gap (exhaust #2, intake and exhaust #3, exhaust #4) to see if they're seating. If they are not seating, I should expect to find rust/crud on the valve stems, preventing the valves from closing, which explains why the gaps got way too big.

My reasoning is this:

- As the engine wears, the valve seats get worn -- they get thinner, so the valve is now rising up higher in the head against the thinner valve seat. That means the valve shim clearance gets *smaller* because the valve is rising up higher in the head as the valve seat gets thinner.

So I should NOT be finding these huge clearances in the valves -- the clearances get SMALLER as the valve seat wears!

The only explanation is the prior owner messed up a valve adjustment and put the shims back in the wrong places, or there is rust on some of the valve stems so they are sticking in the valve guides and not fully closing, making the gaps too big.

OR POSSIBLY BOTH. This bike has been sitting since 1983, 27 years. So 'rust on the valve stems' is plausible, although the bike was not sitting outside for most of that time.

Here's what makes me *really* think that the prior owner got a valve-adjustment all wrong and reinstalled the shims in the wrong spots:

THE COMPRESSION ON CYLINDER #3 IS (marginally) OKAY at 120psi, at least compared to the rest of the bores, despite #3's intake clearance being WAY too high (.229mm, max should be .100mm or less). The exhaust valve clearance is also too big. If 'rust on the valve stem' was suggested to explain why these clearances are way too big ('rust on stem = valve not seating = clearance too big'), I would NOT be getting 120psi.

Let's face it, as the valve seats wear, the clearance gets LESS not BIGGER! How could the clearances on #3 be way too big yet still give me pretty decent compression. Doesn't make sense. If the valve seats were burned that would explain a *larger* gap over time but the compression wouldn't be 120psi.

The original factory-set valve clearance was between .05mm and .10mm, the only way those clearances could get BIGGER while still giving decent compression of 120psi is -- if the prior owner tried a do-it-yourself re-shim valve adjustment and put the shims back in the wrong spots. He might have put too-thin shims in #3, and the valves are fully seating, giving me the (marginally) decent compression of 120psi.


For the rest of the low-compression holes, whose valve gaps are TOO BIG -- those valves may have rusty stems and not seating all the way. That explains why valve clearance reads too big. Valve clearances don't get 'too big' over time. They get 'too small'. Unless the stems are rusty, and/or the prior owner put the wrong shims in.


It's possible that, since the bike is into the 20,000s of miles, the prior owner decided to try to adjust the valve shims and lacked the special tool (that I have) which permits pulling out the valve shims without having to yank the cams. So he pulled the cams to get access to the shims, and then didn't keep track of 'which shim/which valve' and reinstalled the shims in the wrong spot.


OPINIONS AND ADVICE ARE NEEDED! THANKS SO FAR!
I don't want to pull the head. I'm going to pull the exhaust off and the carbs off and see if I can find out which valves are not closing.

Leakdown test would indicate areas of suspicion for insufficient compression.

Good Fortune!

you have done the homework and by seeing that its sat for a extended period of time with one year outside, might as well pull the head, get the valves done, mic to cyls and see if there is any rust that will require boring it out with new pistons and rings.
plus when you tear it down you will know how everything is when you finally do get to ride it.

from your valve clearances i would say someone has been playing about with them ,hence it being left stood ,i would bite the bullet & lift the head .they could have the cam timing out or even nipped a valve .
28 years is a long time so i would fill the engine to the top with kerosene & diesel & leave it standing whilst you do the topend job .
i once freed off a weather siezed small block chevy engine this way & it ran fine for all the time i had it.
it sounds like you know what your doing just get stuck in.
you could just shim the engine up loose & run it to see if it comes back to life then reshim to spec.
i did this on a zxr750 with sticking valves ,went out revved it hard & reshimmed & it was fine .

Another thing that I found when I pulled my head was uneven carbon buildup on the valve seat. I'm pretty sure that, plus the valve stems sticking in the guides was the primary reason for my low compression.

Understand your reluctance in pulling the head, but if you do, check over the cam guides while you are at it. My cam sprockets and chain were in great shape, but the rubber cam rollers and front guide were worn and brittle. I'm saving up to replace those now.