Low compression, weighing options.

Last year, I refurbished my '78 KZ1000 B2 engine. It had 43K miles on it, and had broken cam chain guides. While I was in there, I decided to put on new rings, valve job, etc.

When I got it all back together, I was surprised when I did a compression test that my numbers were all quite low (about 95-100psi on all 4). Did a leakdown test and confirmed that the leakage is into the crankcase (pistons). I wrote about it on this board, and some of you told me not to worry about it, just put some miles on it and give the rings a chance to seat.

Well, here we are 8-9 months and 2000 miles later, and I still have the same compression readings. The bike seems to start and run well. But there's a part of me that just cannot stand the thought that it could be better.

I'm thinking that I must not have done a good job at measuring the cyls and ring clearances before putting it all back together.

So now I am thinking about finding a low-miles set of jugs to replace what I have now, and see if that solves my problem.

So my question is this. If I do that, do I really need to buy a new set of rings again? Or, with just a couple thousand miles on the set that I have on there now, is there a reasonable chance that they will seat properly into the new jugs?

did you install the rings dry? I have heard that if you install wet ( with oil ) that they may not seat,but that does not solve the problem at hand

BSKZ650 wrote:

did you install the rings dry? I have heard that if you install wet ( with oil ) that they may not seat,but that does not solve the problem at hand

Skyman
RE: post by BSKZ650;
Did you use full synthetic "break-in" oil?

BSKZ650 wrote:

did you install the rings dry? I have heard that if you install wet ( with oil ) that they may not seat,but that does not solve the problem at hand

If I recall correctly, I did oil the cylinder walls before putting them on. Not positive, though.

JimatMilkyWay wrote:

BSKZ650 wrote:

did you install the rings dry? I have heard that if you install wet ( with oil ) that they may not seat,but that does not solve the problem at hand

Skyman
RE: post by BSKZ650;
Did you use full synthetic "break-in" oil?

No, I just used standard "mineral" (non-synthetic) oil, which is what I saw recommended by some on this site.

The leakdown test is very good evidence. But offering some thoughts anyhow ---

Did leakdown reveal any exhaust valve leakage? Any intake valve leakage? Are valve clearances set to specs?

Of course the throttle was held in wide open position for the compression test . And does a teaspoon of oil added into the sparkplug hole significantly increase the compression reading? (This would confirm the leakdown findings.) How rapid is the leakdown? Gradual or instantaneous?

Would also consider performing another test with a different compression test tool (maybe the gauge is faulty or the tool is leaking).

I prefer the tester that screws (threads) into the sparkplug hole as opposed to the rubber-tip style that's held pressed into the sparkplug hole during testing.

Just hoping it might be something simple -- like a cheap and easy fix.

Patton wrote:

The leakdown test is very good evidence. But offering some thoughts anyhow ---

Did leakdown reveal any exhaust valve leakage? Any intake valve leakage? Are valve clearances set to specs?

Of course the throttle was held in wide open position for the compression test . And does a teaspoon of oil added into the sparkplug hole significantly increase the compression reading? (This would confirm the leakdown findings.) How rapid is the leakdown? Gradual or instantaneous?

Would also consider performing another test with a different compression test tool (maybe the gauge is faulty or the tool is leaking).

I prefer the tester that screws (threads) into the sparkplug hole as opposed to the rubber-tip style that's held pressed into the sparkplug hole during testing.

Just hoping it might be something simple -- like a cheap and easy fix.

Thanks Patton. All good points. Let me address each of them.

1. No valve leakage evident.
2. Valve clearances within spec.
3. WOT--check!
4. Oil in the hole DID give abt 20% increase in PSI
5. Leakdown rate at abt 15%
6. Gauge gave good readings on other bike, so I think the gauge is okay. Plus, I borrowed another gauge, and got the same readings.
7. My gauge is the screw-in type.

What I'd really like to know, if anyone has knowledge about this, is if the problem is indeed with my cyls being worn beyond spec, and I throw a lower mileage set of jugs on there, will my current (lightly used) rings have a shot at seating properly in the new jugs? Or do you always need to use brand new rings when replacing the block?

Sky,

I'm a noob here to KZ but not to air cooled engine building so here goes:

If you simply replaced JUST the rings, then you should have installed a ring into each cylinder and checked the gap between ring-ends with a feeler guage. This gap of course widens as the rings and cylinder walls wear. This is normally spec'd for engine rebuilding as to what is acceptable gap. Often for new rings, you may even have to shave them down a bit to avoid ring expansion jamming-up and gauling the cylinder walls.

If your cylinder walls were worn excessively (ie: new rings not sufficient), then you should have noticed an abrupt elevation change of the cylinder walls very close to the top (where the rings switch from forward to backward motion during full cycle). Even just dragging your finger nail up the cyl. wall near the top inch, you'd feel the "bump" where it changes from very worn to untouched by ring wear.

Also, I'm assuming that when you assembled your jugs with new rings, that you knew to rotate the compression rings to put the ring gaps about 120 degrees apart? If not, then the compression rings will leak fast. I'm A-type and take this to the oil scraper rings too which I also rotate the top and bottom parts of the oil scraper to be 120 degrees apart too.

HTH.

Like I said, this is not experience gained on KZ's just air cooled engine building I've done lots of.

Thanks, Wrenchmonkey.

Let me tell you what I did do. When I installed the rings, before mounting them on the pistons, I did slide them down into the cyls and measured the gap. As I recall, it did come in within spec. Also, I did not notice any ridge at the top of the cyls, as you described.

Also, I did stagger the gaps, as prescribed in the factory manual, with the top ring facing directly forward, the second ring facing directly rearward, and the oil scraper rings staggered 30* from the oil ring gap.

What I'm thinking I may have missed is that the cyls may have been slightly ovaled. I tried to measure for this before reassembly. I bought a set of cheap T-Bore gauges, and made several measurements of each cyl at different depths and different angles. I found it was very hard to get consistent measurements (meaning I would measure and remeasure in the same spot multiple times, but each measurement was slightly different). And so in the end, I just called it good and proceeded with reassembly.

I tried to be quite thorough to make sure everything was within spec, but I remember that was the one measurement I just couldn't be sure of.

heheh,
I just gotta ask: about yer avatar image; are you picking the chairman's nose or giving him the finger? :lol:

Well, it sounds like ya did everything you could to produce the best results. Failing a momentary lapse of concentration and installing a compression ring upside down or something...
That would only leave a distortion in the cyls as explanation given you KNOW the leak is via the block and not the heads/valves...
But I dunno if ovaling of a KZ's cylinders is common or even normal? In my experiences that would require some heavy abuse to attain and you don't outwardly seem like the break-neck speed demon type likely to run redline all the way to/from work each day

You could always have the cylinders honed and adjust the pistons and rings accordingly but that's a major rebuild and several cigars for your machinist-friend (I'm so lucky to have a machinist-friend that'll work on my "projects" for me at a cost of good cigars! :laugh: )

The thing that bothers me about the oval'd cylinders idea is that your compression readings are all equal or at least within 10% right? Now what are the odds of that if ovaling of the cyls is suspected?
More things to make ya go "Hmmmmm?"...

wrenchmonkey wrote:

heheh,
I just gotta ask: about yer avatar image; are you picking the chairman's nose or giving him the finger? :lol:

Uh, that's my index finger.

It was a bit scary trying to pose for that one. My wife was taking forever to get it positioned just right, and some uniformed guards were starting to take notice. I kept telling her quietly, but forcefully "hurry up!"

Well, it sounds like ya did everything you could to produce the best results. Failing a momentary lapse of concentration and installing a compression ring upside down or something...
That would only leave a distortion in the cyls as explanation given you KNOW the leak is via the block and not the heads/valves...
But I dunno if ovaling of a KZ's cylinders is common or even normal? In my experiences that would require some heavy abuse to attain and you don't outwardly seem like the break-neck speed demon type likely to run redline all the way to/from work each day

You could always have the cylinders honed and adjust the pistons and rings accordingly but that's a major rebuild and several cigars for your machinist-friend (I'm so lucky to have a machinist-friend that'll work on my "projects" for me at a cost of good cigars! :laugh: )

The thing that bothers me about the oval'd cylinders idea is that your compression readings are all equal or at least within 10% right? Now what are the odds of that if ovaling of the cyls is suspected?
More things to make ya go "Hmmmmm?"...

You are right that I am not the speed demon type. However, I just got the bike last year, and I have no idea how the POs treated her.

I guess I should just tear back into the engine and have a look. But I'm going to wait until this riding season is over. Today the weather is perfectly clear and 70 degrees here in Portland. Rode my bike to work. As long as we still have these kind of days around, I'm not taking her out of service.

is it low compression? induce some pinging to seat the valves and rings........no.

just go flog the thing like theres no tommorow, if that doesnt seat the rings nothing will.

i thought i heard someone say 1075 kit?