Please explain compression ratio

[John Carr]

My RS 125 will not start despite having a good spark. I have been told the rings might have gone so I borrowed a compression tester and it measured 65psi, does a compression ratio of 12.5:1 mean the compression should be 184 psi?

[aReSCXXV]

Hi,

no, that's only the ratio. the compression should be 7 bars (6 min)
so between 85 and 100 psi. yours is a little low...

[jenko]

When i did a compression test a few months ago i got 115 psi and with a teaspoon of 2-stroke i got 135 psi when the engine was fully warmed up.

as ive been told the engine should be warm and the throttle wide open, kill switch on then press the starter.

if you are gettin 65 psi there must been something wrong and if you get that on a cold engine theres def a prob some where?

[aReSCXXV]

Originally posted by jenko
When i did a compression test a few months ago i got 115 psi and with a teaspoon of 2-stroke i got 135 psi when the engine was fully warmed up.


Hi,

i get on mine 125 psi, warm (70 C) and no additional oil...
but then my engine is in excellent condition :)
the values i mentioned are from Aprilia workshop. 7 bars is
considered normal for cold engine, and 6 bars is the lowest.

[Kickstart]

Originally posted by John Carr
does a compression ratio of 12.5:1 mean the compression should be 184 psi?

That is the theoretical compression ratio (ie, total volume compared to unswept volume), but of rather more importance for compression (and still not directly comparable) is the corrected value (total volume after the ports are closed to the unswept volume).

All the best

Keith

[jenko]

maybe that is for a 4-stroke engine.
as the rs engine`s exhuast is connect straight to the barrel?
so compression isnt so high?

[John Carr]

Thanks guys. Looks like its barrel off time. Is this an easy job? I`ve got an exploded picture of all the bits and pieces and am thinking of having a go - you know just start undoing bolts and that. I did it once before with a 50cc Derbi sender - what do you think?

[aReSCXXV]

Originally posted by John Carr
Looks like its barrel off time. Is this an easy job? I`ve got an exploded picture of all the bits and pieces and am thinking of having a go - you know just start undoing bolts and that. I did it once before with a 50cc Derbi sender - what do you think?

Hi,

could be only the rings, as they go before the barrel,
and you lose compression. if you change the rings only, CAREFULLY clean
the piston ring grooves. you should get all deposits out, but without
enlarging the grooves in the process.
if you did it on 50ccm, it's a very similar job. things to remember
when you change the rings: new gasket set, new top end bearing, new
clips for securing the piston pin !!!