psi in cylinder test

[dansofield550]

anyone know what sort of psi figures i should be look for in a gs 650 ?

thanks dan

[Normk]

Gosh, Dan, no one seems to have answered after all this time?

Bikes typically run between 120 and 175 PSI for cranking compression although running compression would be higher. Some variables will occur depending on the type of compression tester as the compression tester adds volume to the compression chamber which, of course, reduces indicated pressure. If recollection is correct, my GS850 has about 130-135 PSI when cranking a cold engine. I would expect 5 - 10 PSI lower if a strong young guy were using the kick starter and as for pressures when being kicked by a chubby over sixty...not even going there. (VBG)

My I inquire as to the reason for the question?

Are you doing a compression test on a particular machine?
Diagnosing some particular problem?

There are many myths and much misunderstanding regarding compression testing. These vary from the procedure required:

1) The engine needs to be cranked/turned at close to a normal cranking speed in order to derive a useful pressure indication. Running a compression test with a poor performing starter/battery will typically result in low compression pressures.

2) Not much use trying a compression test on a cylinder which has been fuel washed. If the cylinder has been flooded, overheated, or left sitting for a long time then there may be virtually no oil on the cylinder walls & compression rings to support sealing. Compression pressure will be lower than the actual pressure which will be achieved under normal conditions. Meaningless reading.

3) Tight valve clearances and other issues will result in low compression.

4) Test with wide open throttle and make sure that the air filter isn't plugged solid. I also recall pointing out to a fellow that having masking tape over the carburetor air horns is a "no-no" for good compression testing. (VBG)

5) A compression test is useless as an indication of oil ring condition.

6) And so on....

A more useful test is the leak-down test if one has the facilities. Ironically, assuming one has a shop air compressor, a leak-down tester can often be fabricated at far less than the cost of purchasing a decent compression tester.

One useful rule of thumb is that a cylinder will not fire if compression is less than 100 PSI. There are a few exceptions but I don't recall any motorcycle engines in the exceptions.

If someone expresses an interest I will try to find an old piece I posted elsewhere on compression testing.

HIH

Norm