Cylinder leaking

I just had the valves replaced in my 1981 KZ1100A and I thought I'd check the compression before I reinstalled the carbs and exhaust pipes. Cylinders 1-3 were all between 105 and 115 lbs (cold) but #4 only got to 70. I did a leak-down test and 1-3 leaked very slowly (past the rings, I think) but with #4 it there is so much air coming out of the exhaust port it feels like the exhaust on my shop vac. I can see the valve and it's closed. The strange thing is that when I open the valve slightly there is noticeably less air coming out of the exhaust port.

I tried sealing the exhaust port and air (a lot but not as much as before) now comes out through the tube coming from the air suction valve. When I plug that, air (still a lot) comes out around the cam end plug near the #4 exhaust valve.

This is the first time I've done a leak-down test but I've repeated it 10 or 15 times since yesterday with the same results. I also pressurized the #4 cylinder while I rotated the crank completely around and at no point was the cylinder close to being sealed. The only things I can think that might be causing the problem are that the valve seal for the #4 exhaust valve is not installed, or there is a leak in the head gasket. I'm not sure how a leak in the head gasket would cause a significant amount of air to come out of the exhaust port unless the valve seal were also missing. Does anyone have any ideas?

KYKZ1100 wrote:

I just had the valves replaced in my 1981 KZ1100A and I thought I'd check the compression before I reinstalled the carbs and exhaust pipes. Cylinders 1-3 were all between 105 and 115 lbs (cold) but #4 only got to 70.

That's pretty bad. How many miles on the engine? My 750 had 77k on it and was holding 120psi stone cold on the cylinder with decent valves. The one at 70 is seriously sick.

KYKZ1100 wrote:

I did a leak-down test and 1-3 leaked very slowly (past the rings, I think) but with #4 it there is so much air coming out of the exhaust port it feels like the exhaust on my shop vac. I can see the valve and it's closed. The strange thing is that when I open the valve slightly there is noticeably less air coming out of the exhaust port.?

That's bizarre. Is it possible the head is cracked?


KYKZ1100 wrote:

The only things I can think that might be causing the problem are that the valve seal for the #4 exhaust valve is not installed,

If you mean the seals on the valve stems, they don't seal compresssion. In fact, they don't seal anything since they do let some oil flow past to lube the valve.

KYKZ1100 wrote:

or there is a leak in the head gasket. I'm not sure how a leak in the head gasket would cause a significant amount of air to come out of the exhaust port unless the valve seal were also missing. Does anyone have any ideas?

Are you sure it comes out the exhaust port as opposed to the head/cylinder gap below the port perhaps?

Bounty hunter,

The odometer shows 21,000 but it wasn't working when I got it. There's no obvious crack in the head, either on the outside or around the exhaust valve. I'm sure the air is coming out of the exhaust port. Since my first post I poured a little oil in the exhaust port with the valve closed and the air seems to be coming past the front edge of the valve where you can't see it from the exhaust port.

KYKZ1100 wrote:

Bounty hunter,

The odometer shows 21,000 but it wasn't working when I got it. There's no obvious crack in the head, either on the outside or around the exhaust valve. I'm sure the air is coming out of the exhaust port. Since my first post I poured a little oil in the exhaust port with the valve closed and the air seems to be coming past the front edge of the valve where you can't see it from the exhaust port.

If you are leaking past a valve, did you check the clearance on it to make sure it's closing? If there is proper clearance, and you can still get compression blowing past a closed valve, the valve must not be fitted to the seat correctly.

Bounty Hunter,
The clearance was .08, but I agree that's the first thing I should check after I get the valve cover back off.

KYKZ1100 wrote:

Bounty Hunter,
The clearance was .08, but I agree that's the first thing I should check after I get the valve cover back off.

The only scenarios I can imagine that would make a "closed" valve leak that badly right after a head job are:

1) Didn't cut the valves/seats, just dropepd the new valves in as is

2) Cut the valve seat wrong

3) mixed up the valves during assembly and got the wrong valve in the seat.

4) Maybe the valve guide got twisted and the valve is no longer perpindicular to the seat.

A valve has a monster spring force driving it into the seat, so a valve that was fitted right to the head should be sealing dead tight.

KY... Who replaced the valves for you? It sure sounds like they didn't grind #4. The length must be between an upper and lower limit in order for the valves to clearance properly. Most valves come a little long and almost always need to be ground to length. I think a good machine shop should have a valve stem grinder. I don't think this is something best done with a die grinder or some generic tool. They should also have touched up the valve seats at a minimum. The old valve wore a pattern into the seat and a new valve just won't fit exactly into that pattern. Someone once told me this phenomenon is called concentricity. In other words the new valve won't fit into the footprint of the old so the seat needs dressed with at least the 3rd stone; maybe not a full valve job depending on the valve seats. Again, this is standard business for a machine shop.

Hope you chose a reputable shop. I would take the head back and let them make the situation right. Last thing, which I am reluctant to bring up... if you installed the cam chain incorrectly, you may have bent the exhaust valve. It don't take much...

Bounty Hunter and WG,

The guy I took it to has a shop in his garage but seems to know what he is doing - he has an industrial milling machine and industrial lathe, among other things, and seems pretty familiar with them. I never asked him but he seems to be a retired tool & die maker. He said he ground the seats and lapped the valves. He did a Honda head for me a few years ago and seemed to do a good job.

Anyway, I took the head back off and took it over to him today. There was what looked like a flake of paint or something between that valve and seat. He scraped it off the valve seat with his fingernail then he relapped the valve. He also checked to make sure the valve wasn't bent. I'm not sure what the test is but after he reassembled it he poured gas into the exhaust chamber and watched it for a couple of minutes and none came through the valve, then did the same with the intake on that cylinder.

I'm going to reinstall the head now so we'll see how it works. Thanks for your comments. I was pretty careful with the cam chain and made sure the crank turned smoothly all the way around several times by hand before I tried the compression check. This time I'll just do the leak down test first.

KYKZ1100 wrote:

There was what looked like a flake of paint or something between that valve and seat. He scraped it off the valve seat with his fingernail then he relapped the valve. He also checked to make sure the valve wasn't bent. I'm not sure what the test is but after he reassembled it he poured gas into the exhaust chamber and watched it for a couple of minutes and none came through the valve, then did the same with the intake on that cylinder.

That sounds good, I'll bet the problem is fixed.

normally you fill the port with solvent or something like it anyway then blow compressed air around valve and seat to make sure theres no bubbles in the port.

I got the head back on and pressurized the cylinder. It leaks down slowly now the same as the other three with no obvious leak out either the intake or exhaust valves. I'll get the rest of it put back together tomorrow and see how it runs. Thanks for all your help.

Before I put everything back together I did another compression test. Cylinder 4 (the one that was at 70 psi before) was right at 100 with the others 105, 115 and 120 (#1). I put a teaspoon of oil in #4 and it came up to 145 and did the same with #1 and it came up to 150. From what I read 100 psi is marginal and the difference between 100 and 120 is pretty big, but I decided to see how it runs and to ride it this summer if it runs OK and have the cylinders done this winter.

I rode it today after I got everything back together and it seems to run well but is backfiring a lot. The bike is stock except the mufflers are pretty unrestricted, the air suction tubes are blocked off and I did Wired George's mod to increase the voltage at the coils (which worked well). I rebuilt the carbs last summer but did not disassemble the choke plungers. I put in new carb holders last summer and new exhaust gaskets today. The valves are new. Any advice as to what to look at first to fix the backfiring? Could the mufflers have anything to do with it? Should I start by taking the carbs apart and cleaning them again? I cleaned everything really well last summer but the FSM says not to dip them so it's hard to be sure everything is clean. I haven't synced the carbs yet.