Compression issues????

Hello, new to this board and wanted to ask a question. I own a few KZ's and just recently picked up an 85 ZN750. The bike was rough but it ran good. The first fix was an exhuast leak from 2 broken stud's, so I removed the head replaced the studs checked rings, they were good, cleaned everything and replaced all gaskets. Got it all back together and it wouldn't start??? So I did a compression test. It read 90, 90, 120, 30...F...!!! I didn't check the valve clearances due to it being 4:30am and I was ready to beat it with a hammer, so I just walked away. I've done a few top end rebuilds with no problems but this one is pissin' me off!! So before I go back to this I was wondering if anyone has any suggestions on what I should be looking for if the valve clearances are in spec?
Thanks in advance.

Do a dry and a wet compression test. If the wet test reads normal or close to it, it is probably rings rather than valves. But it is probably a good idea to check valve clearance regardless.

PFC FNG wrote:

Do a dry and a wet compression test. If the wet test reads normal or close to it, it is probably rings rather than valves. But it is probably a good idea to check valve clearance regardless.

So, I did the wet and dry compression test, and no change? Still going to check valve clearences later. I'll keep you posted.:blink:

Welcome to KZr Born Dead.

Two things come to mind on your bikes prob.
You may have a burnt/damaged exhaust valve or valve seat, or during the reassembly of the top end you just might have broken one of the piston rings on the cylinder that's now showing a very low compression reading.

Since you stated it was "running good" aside from the exhaust leak prior to you pulling the head, I'm leaning towards it being damaged rings, especially with the scarey-low compression reading you're getting on that cylinder. :blink:

With the compression test results, I would guess valves or something along the lines of what Ping said a few minutes ago. He knows more about it than I do.

RetroRiceRocketRider wrote:

Welcome to KZr Born Dead.

Two things come to mind on your bikes prob.
You may have a burnt/damaged exhaust valve or valve seat, or during the reassembly of the top end you just might have broken one of the piston rings on the cylinder that's now showing a very low compression reading.

Since you stated it was "running good" aside from the exhaust leak prior to you pulling the head, I'm leaning towards it being damaged rings, especially with the scarey-low compression reading you're getting on that cylinder. :blink:

Hey, thanks for the input Retro. I was thinking the same thing, but before I took the cylinder's off, I replaced the head gasket cleaned everything and assembled. Did a compression test and realized the low readings. So I tore it down again checked everything and rings seemed to be good. Re-assembled and still getting the same readings, wet or dry? It could be a ring but everything seemed to be within spec's. It's got me a little baffled?

Do a leakdown test. It will not only tell you how much it is leaking, but where it is leaking from.

Jay

APE Jay wrote:

Do a leakdown test. It will not only tell you how much it is leaking, but where it is leaking from.

Jay

Yep.....I second that motion. Fastest way to an answer by a long shot....

Born Dead wrote:

Hello, new to this board and wanted to ask a question. I own a few KZ's and just recently picked up an 85 ZN750. The bike was rough but it ran good. The first fix was an exhuast leak from 2 broken stud's, so I removed the head replaced the studs checked rings, they were good, cleaned everything and replaced all gaskets. Got it all back together and it wouldn't start??? So I did a compression test. It read 90, 90, 120, 30...F...!!! I didn't check the valve clearances due to it being 4:30am and I was ready to beat it with a hammer, so I just walked away. I've done a few top end rebuilds with no problems but this one is pissin' me off!! So before I go back to this I was wondering if anyone has any suggestions on what I should be looking for if the valve clearances are in spec?
Thanks in advance.

Look for valves that need lapping.

Born Dead wrote:

It's got me a little baffled?

Take the head off and flip it upside down and put it on the bench. Take some motor oil and thin it down about 50-50 with mineral spirits and pour that into the head where the valves are.

Watch the intake and exhause ports to see if the oil is running past the valves. The ones that leak the oil are the ones that are dumping compression.

If the guy did a compression test dry and then poured oil into the cylinders, not sure I would bother with a leakdown. Seems like I would want to check valve clearances FIRST to see if they were hanging due to lack of clearance. If I found clearances, even if a little tight, I would then consider doing as BH said... about the only-est thing that will cause a compression loss is valve seal or perhaps a messed up head gasket. In a shop environment where a leak down tester and experienced leak down tester operation are present, the leakdown is a MUST because estimates must be given but if you are a do-it-yourselfer you can come to the same place by the normal compression test and the addition of some oil. There is also the dim possibility that compression loss IS due to lack of ring seal as the piston to bore clearance could be so great even a teaspoon of oil doesn't help but this type wear is hard to get since the bike will usually quit starting long before this wear could occur; that is, wear that can't be sealed by a dose of oil.

wiredgeorge wrote:

If the guy did a compression test dry and then poured oil into the cylinders, not sure I would bother with a leakdown. Seems like I would want to check valve clearances FIRST to see if they were hanging due to lack of clearance.

George is right, I thought he had checked that already. If not, definitely do that first.