TOPIC:

on my 1978 Kawasaki z650 b2 I measured the compression losses of the cylinders to check the health of the engine.compression losses are 10% in cylinders 1 and 4 and 7.5% in cylinders 2 and 3.I can say that the engine is still in good health and that the 34,000 km marked are real?

Compression is not measured in % of loss.  Are you referring to a leak down check?  And regardless, it's not possible to judge mileage based on compression.

I am convinced that between the cylinder, piston and valves there cannot be a perfect watertight seal, in fact on new engines there is always about 5% compression loss, so by measuring the compression loss you can evaluate the health of the engine, in fact with this system it is possible to understand based on the measurements if there are problems at the level of piston rings or valves.
ultimately this method is more reliable than the compression measurement.

Yes. In the U.S. that is called a leakdown test. It can indicate the relative health of certain components, but is not indicative of mileage. A low mileage engine that has been abused can leak more than a high mileage engine that has been well cared for.

wrote:

Yes. In the U.S. that is called a leakdown test. It can indicate the relative health of certain components, but is not indicative of mileage. A low mileage engine that has been abused can leak more than a high mileage engine that has been well cared for.

Agree completely, VW and Audi vehicles in the 80's used to use oil like crazy for the first 10k mls then hardly use a drop even at 100k mls and more once everything was "bedded in" (the owners handbook quoted 1 ltr per 1000km as being ok  )

wrote:

wrote:

Yes. In the U.S. that is called a leakdown test. It can indicate the relative health of certain components, but is not indicative of mileage. A low mileage engine that has been abused can leak more than a high mileage engine that has been well cared for.

Agree completely, VW and Audi vehicles in the 80's used to use oil like crazy for the first 10k mls then hardly use a drop even at 100k mls and more once everything was "bedded in" (the owners handbook quoted 1 ltr per 1000km as being ok 

)

One of high school in 1979 I pumped gas for a period of time, and when a VW Rabbit (Golf) would come in I knew that I could sell them a quart of oil.  These cars were not low mileage per say, but they weren't beaters either.  I don't know why they burned so much oil, I just know they did.

Dmax68, I think you are generally correct, but first of all you need to know the compression when the bike is new.  Secondly, you need to measure it under exactly the same conditions every time.  This means temperature, air pressure and humidity, etc must be the same.  Lastly, you need to use the same gauge to measure, as there is considerable difference from gauge to gauge.  Ultimately I don't believe it's possible to be more accurate than +- 5% unless all conditions above are satisfied.

I don't have a tool to check the compression and I needed to make sure the valves were tight, so the only way to check was to pump 80 psi air when the piston is up and the valves are closed.
in these conditions the instrument was reading between 72 and 74 psi so I can be sure that the valves are well closed.

How quickly did the pressure go down?

when I put pressure on the combustion chamber it did not go beyond 72/74 psi and the air supplied was at 80 psi.

If you don't have a tool to measure compression what are you using to measure 72/74 psi?  

If both gauges are accurate, then that amount of leakage is not bad and it should be very easy to hear where it is escaping. 
This video shows how a leakdown test works. You do not have to remove the carburetors or exhaust like he did to make the video .