New piston/rings?

Hi. I have a Z200 which does not idle when warm. It has white smoke when starting up, goes away eventually, but smokes white when riding. I assume the valve stem seals are very bad. The plug is black. Doubt this would automatically only be a possible rich mixture. I'm waiting for new seals to arrive. The compression when riding seems great, there's power in heaps. A compression test however only reaches around 60 psi. If a new set of rings is needed, do I order a size up or is that only necessary if the barrel is buggered and needs skimming? Thanks

Your compression is about half of what it should be. Sounds like you need valve seals but you may not need new rings. You can get an indication of how good your rings are by doing the following.

First, check compression normally...with a warm engine and the throttle held wide open. Then put a few cc's of oil in the cylinder and measure compression again. Compare results to the first test. If the two are close to each other then your rings are probably fine. But if the second test is considerably higher than the first...you probably need to change your rings.

If you need to change your rings you will have to to check the specs on cylinder and piston before deciding how much work is necessary to return the engine to normal operating parameters.

I always question the idea of putting oil in a cylinder to try to discover if the rings are bad. Here's why ......







I believe adding any oil to a cylinder must automatically increase the compression ratio. This is because the oil cannot be compressed so there is less compressible volume in the cylinder, so the compression ratio will naturally increase as oil is added. Ed

I believe a better test than adding oil to the cylinder would be a leakdown test.

A leakdown test injects air pressure through the spark plug hole to pressurize the combustion chamber.

There's a test kit that includes a fitting to inject air into the spark plug holes, along with pressure gauges to determine rate of compression loss, etc.

Air heard escaping from exhaust port indicates exhaust valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from carb intake indicates intake valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from crankcase breather indicates loss of compression past rings into crankcase (perhaps worn piston rings or cylinders).

Air heard escaping from head gasket area indicates loss of compression past head gasket (perhaps due to a blown head gasket).

Poor man's leak down test --

Can perform a poor man's leak down test by introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber. This may be done at TDC or any other cam positions where both valves are fully closed. Top dead center (TDC) is when the piston is at its highest position. An easy way to tell TDC is by alignment of the T mark with the case mark when viewed through the timing window.

May introduce a spurt of compressed air by using a rubber cone-tip blow gun (rubber air nozzle) held into the spark plug hole. And of course keep holding it in position to keep air from coming back out the spark plug hole while listening for escaping air at other places. The air compressor should not be running while listening because the noise will likely drown out any sound of escaping air.

Air heard escaping from exhaust port indicates exhaust valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from carb intake indicates intake valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from crankcase breather indicates loss of compression past rings into crankcase (perhaps worn piston rings or cylinders).

Air heard escaping from head gasket area indicates loss of compression past head gasket (perhaps due to a blown head gasket).

A leaking valve may sometimes be resolved by adjusting the clearance to within specs.

A leaking head gasket may sometimes be resolved by torquing the head fasteners when the engine is stone cold (such as after sitting overnight).

You can generally borrow test equipment from a major auto parts store such as Auto Zone or one of its competitors. Ed

I agree that adding some oil will decrease the combustion chamber volume, thereby increasing the compression ratio. However Ed's example greatly exaggerates the amount. Rather than using the numbers in his example why not use the real numbers?

The bore and stroke are 66mm by 58 mm, resulting in a swept volume of 198 cc. The standard engine compression ratio is 9/1 which means the head chamber at TDC is 22 cc. Assuming ALL of the 5 cc of oil remains on the top of the cylinder (unlikely) it would result in a decrease in volume to 17 cc. The compression ratio with a swept volume of 198 cc and a chamber volume of 17 cc is about 11.6 , compared to the original 9.0.

Back to the oil placed into the cylinder....some or most of it will flow to cylinder walls and coat them. This means the entire 5 cc is not sitting on top of the piston. Which means the maximum compression caused by less volume is less than 11.6.

I also agree with Ed that a leak down test is a better way to check. I didn't mention it because most people don't have the ability to conduct one.

hardrockminer wrote: I agree that adding some oil will decrease the combustion chamber volume, thereby increasing the compression ratio. However Ed's example greatly exaggerates the amount. Rather than using the numbers in his example why not use the real numbers?
.............

I agree that the example I gave was generic; it was only intended to show that adding oil to the cylinder will increase the compression ratio. It shows a 100cc cylinder which is considerably smaller than that of the larger bike engine in the original posting, so the 5cc oil amount has a greater impact on the compression ratio in the example compared to the actual engine in question. Your example shows a similar but more specific result as its dimensions are much closer to that of the engine in question. Both your specific example and my general example show that adding oil will raise the compression ratio.

Regarding the leak down tester - I don't know for sure if this tool is available as a loaner tool, but it is probably worth checking at Auto Zone, O'Reilly, Advance Auto, etc. to see if it can be borrowed as a loaner tool. I know for sure that those places have other tools that they loan out, but they may or may not have this specific tool that can be borrowed. Ed

The numbers I used are actual specs for that engine.

However in this case the actual compression is half of normal. Since the "oil in the cylinder" trick is for indication purposes only I believe a dramatic rise would suggest ring damage and a small rise would suggest the rings are fine. It's a very easy test to do.

I've never done a leak down test but I can imagine it's a bit more complicated because you need to rotate the crank to a position where both valves are closed.

hardrockminer wrote: I agree that adding some oil will decrease the combustion chamber volume, thereby increasing the compression ratio. However Ed's example greatly exaggerates the amount. Rather than using the numbers in his example why not use the real numbers?

The bore and stroke are 66mm by 58 mm, resulting in a swept volume of 198 cc. The standard engine compression ratio is 9/1 which means the head chamber at TDC is 22 cc. Assuming ALL of the 5 cc of oil remains on the top of the cylinder (unlikely) it would result in a decrease in volume to 17 cc. The compression ratio with a swept volume of 198 cc and a chamber volume of 17 cc is about 11.6 , compared to the original 9.0.

Back to the oil placed into the cylinder....some or most of it will flow to cylinder walls and coat them. This means the entire 5 cc is not sitting on top of the piston. Which means the maximum compression caused by less volume is less than 11.6.

I also agree with Ed that a leak down test is a better way to check. I didn't mention it because most people don't have the ability to conduct one.

And also least anybody should forget. The oil will migrate out of the cylinder in the same way as the rest of the cylinder pressure.
For anyone who has added oil on a compression test knows. A follow up test right afterwards will show a loss back to the previous
pressure before the addition of oil to the cylinder.


.

This clip is out of the Kz1000 FSM. Says nothing about adding oil... never understood that... if you have low compression then you can do a leak down test. I’ve done it with just a regulator and compressed air. You need to hold the crank at TDC, apply air slowly till you get to 100 psi. Then listen for air leaks... exhaust, crankcase, carbs... don’t really need a leak down set.... but there cheap and a descent tool to have... the wife always needs more shoes!!!

You can build your own leak down tester.
Mike Nixon has the plans on his site
www.motorcycleproject.com/text/Leakdown.html

The one I built is the link below. I think it may have been $40 (picture of mine is attached)
You just need to nail the orifice restriction.
s14net.vbulletin.net/forum/s14/general-e...spec-leakdown-tester


Mobuzz,
Have you adjusted your valves?

Hi. I've checked them, the clearances seem fine. This is however a project bike which I bought, as well as a parts donor bike which is a later model. I have no idea when it last ran before I bought it. I doubt anything has been done to the engine i.t.o maintenance since '82. The donor bike engine was already split open because the previous guy swopped the head to his project bike. I transferred the electronic ignition of the newer bike to the older (removed the points), to get it running and to establish where and what would need replacing, instead of overhauling the whole engine. Parts are difficult to come by, more like impossible to find here in SA. so didnt want to spend a fortune on unnecessary parts, poor Rand / Dollar exchange rates and 20% import duties. Chances are I'd have ordered incorrect stuff as well, since this is my first attempt at a restoration/rebuild. I've already made a few expensive, less ideal purchases out of lack of kowing better. I'm hoping the piston and rings dont need to be replaced since there are different size options, ordering the wrong one would be a mission to fix (return shipping to US or UK, time for shipping, more import fees etc)