Aprilia RS 125 low compression, help!

jamesr5 · L Plate Warrior Joined: 07 Jun 2012 Karma :

right okay, I just bought an RS 125 for £1000, it's a 2000 reg and just had an engine rebuild about 770 miles ago. I'm facing some difficulties in diagnosing a problem, it seems the engine is giving a compression of 118psi when cold, yet when the engine is warm it gives a compression reading of 110psi. so basically, the engine starts when it is cold, you get it warm and go for a ride and it loses compression and dies. very annoying when you live on top of a hill, and it dies at the bottom of the hill... a lot of effort let me tell you ahaha. But yes, I think it may be the piston rings as I heard a slight whistling from the engine once, but thought it was me being stupid as it was working perfectly fine at this point and carried on for about 20/30 miles working perfectly fine. I'm about to take the engine out and have a look inside, see what is up, but just thought I'd see what everyone thought it was, as i'm 17 and have never taken an engine apart, only dealt with basic mechanics on my old bike and a lot of bodywork experience, nothing with engines though! I will upload pictures later in the thread when I get the engine out

HD · World Chat Champion Joined: 16 Dec 2009 Karma :

Any news on what this was?
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Rusty '02 Vito Camper + CBR600F3

RSJames · L Plate Warrior Joined: 14 Jul 2012 Karma :

I also have an rs same year, it could be many things but I would guess rings or the actual piston its self. You don't have to take the engine out I would start by removing the cylinder head and the barrel and looking at the pistons condition. Let me know how it goes.
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Currently riding an aprilia RS 125 full power, yes it has blew up before.

MaybeGuy · Super Spammer Joined: 12 Mar 2007 Karma :

Sounds like you've got a crack somewhere that's getting bigger as it heats up.
Shouldn't it be something like 140psi anyway?
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Blue_SV650S wrote: it was a sh1te wheelie, but it proves that he can get it up in 3rd and can do angles. In summery, mattsprattuk is a gobby little sh1tebag, dopehead tw4t, but sadly for all of us, he probably isn't THAT full of sh1te!! Mr. Green

Kickstart wrote: Hi I tend to agree with Matt. All the best Keith

iooi · Super Spammer Joined: 14 Jan 2007 Karma :

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Would still expect it to start at 110psi. Gets below 100 and you will have big problems though.

All the best

Keith
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symonh2000 · Crazy Courier Joined: 20 Feb 2005 Karma :

There isn't exactly much to go wrong on a single cylinder 2 stroke.

Most like to be the ring, barrel or the piston, but I would have though that if the power valve was stuck in the up position it could reduce compression slightly as well, as you will trap less gas in the cylinder.
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Four strokes is two strokes too many.