change the rings..... and or piston and or little end bearing etc... depending on age.

 

id leave it till it reads 110 or even 100 120 nots bad tbh 90 is a defo DO NOT RUN THE BIKE THO

 

I would swap the rings. Whats the piston to bore clearance ?

 

Honselty not a clue, not had the bike too long and had had an engine rebuild before i got it, all i know is its a prox piston lol

Also whats a good compression reading then, what should i be looking at?

 

I really need to rewrite this thread and get it stickied, ive posted a link to it three times today. lol

http://www.125ccspor...showtopic=75114

 

120's borderline, probably agree with pego, should be ok for a while, although if you've got money and time to do the piston/rings then go for it.

 

weird thing is on a mates rs, done a comp test and got a reading of 105, changed the rings and still got 105, even after rings :S

 

Probably got a Haunted RS125 tbh, will be low compression for ever. Might even rattle like jacob marley as well. Best to get an exorcist.

 

worn crank seals effect the compression

 

120 is just fine and dandy

I have 2 brand new bikes here, 1 with just over 1000 miles on it and one with 80 miles. Both have 120-125 psi depending on what gauge i use (Snap on V Draper tools one)
A compression tester is just a tool, the main use is on 4 cylinder engines to test or diagnose problem. IE if you have 140 psi on 3 cylinders but 90 psi on 1 then you know you have an issue on that cylinder.

How the tool is best used on a single cylinder engine is to check the compression and make a note of the psi, then check it every now and then, if you get a reduction then you know to investigate.

It is also worth noting that not all compression testers read the same.

Hope this helps

 

^^^^^^^^^ Can't argue with that advice ^^^^^^^^^

 

What would you swap the rings at Paul ?

Cagiva Mito's run to about 120 psi after 8k ?

 

Mito has a diff bore and stroke so will not be the same as the RS125

And don't forget the reading is from the top of the exhaust port so the "psi" will vary from engine to engine IE Mito, DT125R, RD125 etc.
With the RS125 i have seen bikes with 90 psi pull really hard, and produced good power so hard to say. If the OP has 120 now and say in 1500 miles time he has 100 then i would tear it down and check the ring gap.

 

The last mito i saw with 85 psi compression had the rings melted flat with the piston. heh

 

First question I would ask is - how good is your compression tester? Because I once had a cheap one that gave me a compression reading of 80psi, compared to a Gunson one that read 150psi on the same engine on the same day.

 

My compression tester gave me 210, on a working one it was 140 lol.
Mine was made in the stone age though tbf.

 

I use two different Gunson testers, both seem pretty reliable tbh.

 

I did tested mine the other day with a cheap tester, and came up with 120psi "ish"

Last owner put a new set in though, so going off whats been said here, i'll just leave it

last owner put them in just before i got the bike, unless he lied i guess

 

120psi is ok, but I would be looking for new piston rings.

Changing the rings more often is so much cheaper and easier than changing the whole top end when the engine seizes.