TOPIC:

Hello All,
I have a KZ550 LTD model, it has 38k miles on the engine.

I did a compression test last week, Cylinders 1,2, and 4 are all around 140 psi. However cylinder 3 is around 70 psi. The oil was drained and the engine was cold. So I added some oil down cylinder 3 and the psi jumped up to 120 psi. Bad rings.

The bike always idles rough since the day I got, with 25k miles. I have to leave the idle above 1500 rpm. I never once was able to turn on the bike without using the choke from a cold start. Even when its 90 degrees outside.

I recently rebuilt the crabs back to spec, In 1,2,3,and 4 gear the bike accelerates nice and smoothly. However in 5 or 6 gear it SOMETIMES has trouble. It is like when I give it throttle I go nowhere the engine just chokes up. Also when going 70 mph in 6 gear when I do WOT I literally go nowhere and after like 30 seconds the bike actually brings to go faster then 70 mph. The bike was doing the same thing BEFORE i rebuilt the carbs. I have a feeling its because of cylinder 3 having low compression.

Another thing i also noticed is the carbs don't have the correct fuel level when the petcock is on "ON" or "RES". The air tube that connects to the carbs goes to cylinder number 3.

So, I am thinking that cylinder 3 is pretty much gone by now. What do you guys think??

Oh last year I checked the valves clearances too, it's in spec now. I just know someone was going to say check the valve clearances, lol.

I have a spare 550 motor for a A3 Model. I am planning on removing those pistons and rings and installing them into C3 motor. Should I only exchanges the pistons and rings. Or also included the A3 block too?

I have a 1980 KZ550 LTD and it's harder to start cold than any other bike I own. Carbs need to have floats properly set.

Not much to say about low compression in one cylinder. If you decide to tear the engine down to change rings in one cylinder you should check the cylinder for proper dimensions before installing new rings. You should also prepare it correctly...hone the cylinder before reassembly. I would not switch in a piston from another engine without checking dimensions.

70 psi is certainly in need of attention. Since your top end would be coming off to replace rings, you can adress any issues with the head and valve shims on reassembly. Then you will be able to chase down remaining issues in the carbs with a good basic top end rebuilt.

Before you make a bunch of plans to replace a piston or rings, you really need to do a leak down test. A compression test only tells you that you lack compression, but a leak down test will tell you whether it is from a bent intake valve, burned or bent exhaust valve or if it is indeed bad piston rings. Leak down testers are expensive and are a tool that you will rarely ever use, so sometimes they can be rented from auto supply shops or you can do what I did and just make one using the hoses from a compression tester and misc parts from the hardware store.
I’m not going to bother explaining the whole testing process as it’s easily found with a quick Google search.
Moral of the story is you just may need a valve job as opposed to deeply involved cylinder block and piston work.

Pay attention to the comments by Daftrusty! His advice is correct, and you cannot perform a leak down test after you pull the cylinder head. Ed

He has put oil down the bore which increased compression and that is a strong indicator of ring or bore issues.
If it was bent or sticking valves there would not be an improvement.

If you plan on using parts from your spare motor you must use the corresponding cylinder too to have half a chance of getting a usable top end.
Used rings from one will not seal in the other.
Your other option is to hone the cylinder and fit new rings if the piston/bore clearance remains in spec..
You may find your low cylinder is just due to stuck rings or blown head gasket etc and the fix may be simpler than you think.

The issue I have with just putting oil in the cylinder is that even if the rings and cylinder are perfect it will still raise the compression. The images below, which assume a perfect seal between the rings and cylinder wall, show what I mean. Even with perfect rings and cylinder walls the compression will rise if oil is added. Please don't misunderstand - his problem may very well be with rings or cylinder wall condition, but a leak down test will show if there are other issues that should be addressed. Ed

zed1015 wrote: He has put oil down the bore which increased compression and that is a strong indicator of ring or bore issues.
If it was bent or sticking valves there would not be an improvement.

This is very good point, but pouring oil into a cylinder with a perfectly good piston/bore and rings will also lead to an increase in compression as it plugs the ring gaps.
So in theory he may have boosted that cylinders pressure making ability by plugging the ring gaps with oil (raising compression) all the while compression was escaping through a leaking valve. Even with oil he still was 20 psi away from the other cylinders. Hence the leak down test will confirm if he has a broken ring or a melted piston. Or maybe it’s just carbon baked onto a valve seat.

Food for thought....

**Edit**
650ed and I made the exact same point, but he must have posted it while I typing this post. Great minds think alike.

I don't have a leak down tester with me. But the spare motor is in good condition before i disassemble the bike. It run well, It only have 16K miles on it.

Anyways when I remove the head, I will only pull off the valves from cylinder 3 to check them. However, I leave the rest alone since I don't have the proper tool to remove the valves. The jerry rigged way I came up with is a pain in the butt.

I am planning on doing this rebuild in a day. just waiting for the gaskets to come on Monday. I will measure the rings for all pistons and select the best ones. Also bought a cheap honing tool.

I also forgot to mention that I THINK I am hearing a clicking valve when the engine is higher then 5k RPMs but I am not sure. I couldn't really tell if it was a clicking valve, But last time i went riding I didn't hear anything. Anyways, the head is coming off one way or another. Already have the gaskets coming in.

I just need this bike to last one more season. Before I upgrade to newer and bigger bike. So far I am proud of this little 550cc motor.

I might disassemble the motor today and see what I will find.

So, Its not a good idea to swap rings or rings and pistons from another motor? Worse case I'll swap the block with the pistons and rings all together, may with the head too.

So if the spare motor is in good running condition.....why not just swap the whole motor in rather than taking two motors apart??

While a leak down tester is a great tool to have in reality it is nothing more than a pressure gauge to measure air line pressure and another gauge to measure pressure loss. The amount of loss is dependent on several things. Once the leak down tester is installed and it is found there is sufficient pressure loss to warrant investigation then you must use your ears to determine where the air is leaking from. Is the air going into the crankcase? Is it exiting via an exhaust port or intake port and so on. For lack of a leak down tester I have just installed an air line with reduced pressure to the cylinder via the spark plug hole and listened if any appreciable amount of air was being lost and where it was coming from. Not the most accurate way of performing a leak down test but in a pinch it can help diagnose a problem. You just can't tell the percentage of air loss. When I have done this I use the hose from a compression tester and remove the Schrader valve from it then connect it to an air supply line. Just remember to reduce the air inlet pressure. You don't need much.
Rick H.