Aprilia RS 125 low compression, help!

[James Reynolds]

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

[Rockynv]

During the rebuild the may have not staggered the piston rings correctly, put the compression ring in backwards, the head/jug may not be torqued right, used the old jug or head gasket, the crankshaft seals could be bad however that should only impact intake since the piston will be past the intake port back to the crankcase during the compression part of the stroke or the reed valve may be defective or in backwards but again I don't believe that should mess up compression that badly. What grade of 2 stoke oil are you using? I think Aprilia specks only synthetic 2 stroke oil for that bike.

[WestOzRS125]

Dramas

Don't pull out the engine, pull off the cylinder only. Check the piston, rings and inside of the cylinder. If there's nothing obviously wrong, do a top end rebuild anyway. If you don't know how to do this then buy yourself a Haynes manual - it makes it easier.

If there is something wrong with one of those parts, find out how it went wrong before you decide what to do next - post those pictures so you can get some help.