Cylinder PSI, Stock 2006 YZ 125

[BNasty]

Anyone know what the psi should be for a fresh set of rings in a stock cylinder? I bought a used bike that is suppossedly bored to 144. My bike is maxing at 175 psi, which seems to confirm the overbore. Just dont know for sure.

On the same note, does anyone out there with a 144 have a fresh ring psi reading for me to work from so that i know when I might need to replace the current rings?

Thanks, in advance.

[GMO397]

My current 250 with fresh rings and cylinder has 250 psi.

My old 125 with clapped out cylinder and fresh rings had around 120 psi.

The air temperature, humidity level, elevation and many more factors determine your compression. Are you testing it with a hot engine?

175 sounds like a good number for a 125. There is no way to verify if it is a 125 or a 144 from the compression numbers. And doing so is a bad idea.

If this is a new bike, I suggest pulling the head and inspecting/replacing the piston and rings.

You can measure the current piston and determine what size piston is in there.

[qwert321]

Numbers depend on gauge(I have a real cheap piece of @%^$%)

2008 yz125 155psi(fresh broken in topend)

same bike athena 144 154psi(fresh broken in top end)

2010 yz125 151psi(fresh top broken in top end)

Put a new piston and ring in the 2008 last topend and lent it to my buddy for a while with out breaking it in. checked compression when i got bike back from him and it was at 160psi after 10hrs. Bike now has 27hrs on that top end and still 160 psi. thinking of letting him break in my bikes from now on.

[BNasty]

Thanks for the replies.

Engine is cold, Im at sea level, temp was in the high 70s, and humidity is high.

qwert321--that is interesting that both your 125 and 144 read close in psi. I thought it would be higher for a bigger piston, but I am no expert.

I think I picked up my gauge at Autozone. I guess the only true way to know if my psi is off is to test after fresh rings/pistion and then monitor as it goes down.

Is there a rule of thumb for percentage drop in psi? For example, if your psi drops X%, it's time to replace the rings.

[qwert321]

I think compression is close because the squish is set at 1.1 mm on the 144. Not sure what stock squish is for a 125 but the Athena 125 cylinder and head is set at0.9mm.

[limp1969]

You can tell by riding it. I hate 144 kits it makes them run like a 250 not a 125 you can hear it when its at idle too.

Better way to make a 125 scream is porting, reeds, pipe, head work race gas and hours of testing and jetting.

[DMauro]

^ you didnt have the right guy do you 144. ericc gores low end 144 just mild port for the bb and the bike rips same powerband as the 125. 175psi is real good any higher and you will need race gas. if it was mine i would run race gas with that bike.

[panicrev]

I've never checked PSI and would love to know how!! Anyone wanna post a pic of measuring tool and/or link?

Is this a good way to measure when you are needing a top end?

[limp1969]

Buy a compression tester sears or what ever

Pull the plug screw the tester in the plug hole make sure it's in good kick the bike over til the gauge doesn't go up any more

There you have it

Like checking tire psi

Very good tool to have to tell the condition of a 2 stroke top end

Every bike I buy is checked before the cash comes out

[BDubb106]

I was usually right around 180 with fresh replate, piston, and rings on my EG Yz144 when I had it.

[DMauro]

^ what gas r u using. just curious.

[Mark6299]

My current 250 with fresh rings and cylinder has 250 psi.

.

That's about what my 2007 YZ250 pressure was at too.

[Mark6299]

FYI. My 2002 YZ125 was 150 and my 2004 YZ125 was 180, measured today. I woder if 150 is low or not. Any ideas?

[2strokenut]

FYI. My 2002 YZ125 was 150 and my 2004 YZ125 was 180, measured today. I woder if 150 is low or not. Any ideas?

Haha you're the badass engineer, and you don't know?

[Mark6299]

Haha you're the badass engineer, and you don't know?

You must have me confused with the other guy. I did not claim to be a badass engineer. A good or even a great engineer is open and often eager to find out about other peoples experience or opinions. This is what I am doing. So if you are calling me a good engineer for these reasons, then I thank you for your compliment.

[limp1969]

That's about what my 2007 YZ250 pressure was at too.

Guys if you are at 250 psi you have a major problem in other words no way

All 125 250 2 stroke should be no higher than 180 and that is high

170 is correct

Psi comp test on a 2 stroke is how you test the condition of your top end when new it should be 155 when broken in 170-175

When it start going down replace rings and piston

[mynewcr250]

I wrench for a living so compression tests are nothing new. However anyone that knows the ins and outs of using a compression tester knows oil helps seal rings. My question is since my only 2 stroke work is on my own bike, how does the premix effect compression tests? I mean IMO the only accurate way to perform a compression test would mean I'd warm my bike up to operating temps, drain the bowl, and then run a compression test.

Seems to me like unless all these are done, comparing compression tests or even hoping yours is even accurate all goes out the window. I would venture out to say my bike running 24:1 is going to have significantly more compression than the guy running 50:1 unless both took premix out of the list of variables.

[TommyB 754]

If wrench for a living then you should know you do compression tests when the bike is cold.

[limp1969]

If wrench for a living then you should know you do compression tests when the bike is cold.

+1 with throttle wide open never warm!!

[limp1969]

FYI. My 2002 YZ125 was 150 and my 2004 YZ125 was 180, measured today. I woder if 150 is low or not. Any ideas?

02 needs a new top end to be 100%