Low compression on one cylinder

Hi anyone

I'm looking for guesses, before I delve into the engine.

I've got a new-to-me KZ440 project. High mileage. It has weak compression in one cylinder (left side).
I think a previous owner tried to chase down a hard-starting and poor-running problem in the carbs, but the real cause was low compression in the left cylinder.

Cold readings:
Right cylinder read 125 PSI.
Left cylinder read 90 PSI.

I checked valve clearances. They were all out. I set them all to 0.2 (within spec). The right cylinder improved slightly, and pumped up to 130 PSI.
The left cylinder still read 90 PSI.

I put a couple of drops of oil down the left cylinder, and it pumped up to 100/105-ish PSI immediately.
I left it alone a few minutes, and tried again. Once again, the left cylinder only pumped up to about 90 PSI.

What do you think? Valves or rings?

I know nothing about the twins but the first thing I would want to know is how long has it been sitting since it last ran well.
you could have gummed up rings on the low cylinder. 

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The bike was sold by a dealer in February, who sold it to the guy who sold it to me just now.

He had the primary chain replaced.

The guy might not be the right person to ask about the bike. He tried his best with the carbs, etc. (even bought brand new ones). But I can tell it's not the carbs, and I was expecting to do major engine work on it anyway.

Gummed up piston rings? You could well be right. But there's no way of knowing, for now. I would have to remove the head and barrels.

Primary chain replacement means splitting the cases, at least it does on the fours, so barrel would have come off. Broke a ring when refitting maybe?

So you put some oil in and compression went up, but not to the equal of the other cylinder.  It sounds to me like you have some valve leakage, and also some ring leakage.

Yeah, these old twins are different in many ways from the fours. I've only had time to look at a couple of diagrams so far, but I'll need to do a deep Youtube dive later on.

Just from what I suspect, the primary drive chain is on the right, and the previous owner removed the clutch cover, saw it, saw that it was slack, and had it changed. I don't think the cases have been split for a long time, on this engine.

Certainly, I needed a powered impact wrench just to remove the rusty old bolts holding on the valve cover, and those bolts had remnants of ancient threadlock on them. It wouldn't surprise me if it were on its original rings and pistons.

Even at 90psi cold, I think it should start. I would check the ignition system, including the battery. A weak battery can turn the starter motor, but not have enough left to fire the ignition system. If the rings are stuck, running it for a bit will loosen them up.

You will find this helpful---  kzrider.com/modules/ServiceManuals/kz440.pdf
 

wrote:

Even at 90psi cold, I think it should start.

 

Sorry for not making it clear.

The bike starts and works. It's just not at its best. It's hard to start insofar as the starter motor has to spin a few more times than is normal. Running through my list of what could cause hard starting, I checked the valve clearances and found they were all wrong. Setting them right and checking compression, the right cylinder is within service limits but the left isn't.

The ignition system is absolutely fine. All components work and it even has new iridium spark plugs (last owner). Strong battery, good starter motor too. I haven't tested the coil but I think it's most likely OK.

Ah, ok. The compression should be measured when the engine is at operating temperature. That and a leak down test should give you a good idea about what is going on internally.

If the valves were tight then they may be carboned up or even damaged.  I wouldn't do anything drastic though until the bike really starts to act up.  Some good hard riding may clean up the carbon and bring the compression up.  Some Marvel Mystery Oil in the gas will help clean up the carbon in the ring lands and valves so that's worth a try for a few tanks too.  If the bike just won't run well I'd go through the carbs and ignition system and update everything you can.  Don't waste money on "carb kits" rather, clean and replace O-rings.  For the ignition replace plugs and maybe wires if it's possible to do so on that model coil.  

wrote:

If the valves were tight then they may be carboned up or even damaged.  I wouldn't do anything drastic though until the bike really starts to act up.  Some good hard riding may clean up the carbon and bring the compression up.  Some Marvel Mystery Oil in the gas will help clean up the carbon in the ring lands and valves so that's worth a try for a few tanks too.  If the bike just won't run well I'd go through the carbs and ignition system and update everything you can.  Don't waste money on "carb kits" rather, clean and replace O-rings.  For the ignition replace plugs and maybe wires if it's possible to do so on that model coil.  
 

Taking it out for a ride sounds good!
I don't mind getting into it, with a piston ring kit and maybe even new valves, but I'm really curious to see what it would be like to ride.

I'm not buying anything related to carbs. lol
I've got a pair of brand new carbs from the previous owner, and a whole bag of jets.
I think replacing both the coil (as it looks like the original one) plus HT leads and plug caps is a good idea.