Low compression on one cylinder?????

Guys, I started messing with the craigslist non running 80 KZ550A. The guy I got it from said he don't know anything about it. The gauges are gone so no clue on mileage. I've spent a couple hours disassembling my push button and run switches and cleaned everything and finally got power to the starter solenoid. I checked the compression with carbs removed. I took them off to clean them. One cylinder has 47 the others are around 135-145. I removed the valve cover and checked the intake and exhaust clearance on that cylinder. My smallest feeler fits .005, the .007 is tight. If this was a valve problem, the clearance would be more right? I'm guessing the rings or a hole in the piston? Anything else it can be? I'm not a bike mechanic but know my way around car engines. So I at least have a little mechanical background. I do not have a manual yet. The cam lobes look OK with no noticeable wear.

Leak Down test may give you some more insight, listen for escaping air in head or crank cases...

Rotate cam lobes where valve should be closed, Penlight through spark plug hole, see light while viewing through intake or exhaust ports......?

Pop the head....
Possible excessive carbon build up on valve seat, damaged guide, broken spring etc... but considering other cylinders to spec I'm wagering a bent valve...

You'd see NO clearance if the valve were tight. If that's the case, it already has a burned seat or it's bent as mentioned above. +1 on the above diagnosis.

Bent valve would by my guess as well.

The right way to figure this out is to pop the top. You might need to pull the cans as well (to check the rings and bore.

How would the valve get bent? Carbon build up cause this? If I tear it apart, where can I buy valve seals and rings? I looked at Z1 but only found a full gasket set for 1980 KZ550A in the engine section

Valves get bent lots of ways. Sometimes, it's as simple as a single over-rev where the clearance was a little too "ample".

Call and talk to the guys at Z-1. They' re really cool. If they know of a top-end kit, they can get it for you.

Will the head come off with engine in bike? It looks like a no go with the studs being tall.

I have only larger bikes, but, Yes. I don't think that head removal is any different. You should be able to pull the whole top end off. Be ready to catch the cam chain tensioner parts.

You may have a bent valve as mentioned, but before you go pulling the head do a leakdown test as OMR stated. It should confirm if the leak is a bad valve; a problem with the piston/ring/cylinder; or even possibly a bad head gasket. If you're not sure how to do the test ask and I'm sure you will get good instructions. Ed

OK, I did not hear any air in crankcase. I removed the head and valves are seated. I put a spark plug in and flipped the head over, I then filled it up with carb cleaner. I let it sit for about 10 minutes and none seaped out around valve. I do not see any damage to piston top. I then removed the jug. No bad scuffs or anything on the cylinder wall. There is no ridge at top if a bike gets em like a car engine does. The head gasket showed no signes of being blown. The jug to trans case gasket was trashed. It was flakey and falling apart. The rings are not stuck in the grooves. I'm at a loss on this. No smoking gun. I cleaned up the combustion chamber on the bad cylinder and see no visible crack. What the heck? There is a lot of carbon build up on the piston tops and especially in the valve reliefs. The bike had a K&N filter in it that I swear someone dipped in motor oil. The airbox for the filter is covered in oil. The big hose going to the top of the air box is completely dry and clean. If the engine had blow by, that hose would be oily inside right?

Forgot to mention, before I tore it apart, I shined a small light in the each end and could not see any light through the spark plug hole.

I removed the top ring and installed it in the cylinder. I do not know what the feeler reading should be on ring gap. I don't have a manual yet. Anyone know? Here's some pics.