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aprilia rs 125 year 2000 compression fault

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ather7on
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Joined: 21 Oct 2012
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PostPosted: 15:28 - 03 Jan 2013    Post subject: aprilia rs 125 year 2000 compression fault Reply with quote
hi, im having problems with the compression and want to know if i had a lack of oil in the mixture would this affect the compression? its just that im not sure if the oil pump is working correctly thats all as i cant start the bike at the minute and simply working the throttle doesnt pump oil out. thanks.
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P.
Red Rocket



Joined: 14 Feb 2008
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PostPosted: 15:32 - 03 Jan 2013    Post subject: Reply with quote
Compression is what it says on the tin.

Compression in the cylinder... the rings press against the edge and as the piston moves up it causes compression. Same amount of air, increasingly smaller space. You want over 100psi, ideally closer to the 110psi+ range in the RS.

How long since it was rebuilt?

Last ring change?

Been stood up a while?

Could probably get away with a piston + ring change.

Worst case, full top end.
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ather7on
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PostPosted: 16:53 - 03 Jan 2013    Post subject: Reply with quote
##Paddy## wrote:
Compression is what it says on the tin.

Compression in the cylinder... the rings press against the edge and as the piston moves up it causes compression. Same amount of air, increasingly smaller space. You want over 100psi, ideally closer to the 110psi+ range in the RS.

How long since it was rebuilt?

Last ring change?

Been stood up a while?

Could probably get away with a piston + ring change.

Worst case, full top end.
so youre saying the oil pump cant affect compression?
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P.
Red Rocket



Joined: 14 Feb 2008
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PostPosted: 17:10 - 03 Jan 2013    Post subject: Reply with quote
If it stops working, it won't lubricate and will cause the piston to score the shit out of the barrel.

The oil pump spins when the engine in running. Thus failing it would cause mass damage.

But checking compression in the head with a broken oil pump should be irrelevant.
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steveh
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Joined: 24 Aug 2004
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PostPosted: 17:36 - 03 Jan 2013    Post subject: Reply with quote
what compression reading are you getting?
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ather7on
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Joined: 21 Oct 2012
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PostPosted: 18:00 - 03 Jan 2013    Post subject: Reply with quote
##Paddy## wrote:
If it stops working, it won't lubricate and will cause the piston to score the shit out of the barrel.

The oil pump spins when the engine in running. Thus failing it would cause mass damage.

But checking compression in the head with a broken oil pump should be irrelevant.
iv measured the piston rings while on piston and inside of barrel and the measurement seems fine compared to book settings, theres no scoring on the inside either. compression read 30psi, changed o rings and gaskets, still reads 30psi, rubbers etc seem fine.
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P.
Red Rocket



Joined: 14 Feb 2008
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PostPosted: 18:02 - 03 Jan 2013    Post subject: Reply with quote
put a bit of 2 stroke oil down the spark plug hole, put the tester on again and try it again.
If you get a higher reading, top end time Thumbs Up Possibly get away with rings, but the piston is a serviceable item.
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thomp1983
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Joined: 05 Feb 2011
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PostPosted: 20:36 - 03 Jan 2013    Post subject: Reply with quote
right size rings and piston for that barrel? is it a stock barrel? is this your bike that has stopped working or a new bike to you that has never worked/recently stopped working?
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pits
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Joined: 22 Apr 2008
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PostPosted: 03:57 - 04 Jan 2013    Post subject: Reply with quote
you want around 150psi of compression....120 and consider rebuilding it.

30 psi, in short she is fucked, like proper fucked, I would imagine new top and bottom end, you have had the head off but no mention (as far as I can see) about replacing the head gasket with the correct size, there are 3 gaskets depending on compression ratio the bike itself has, very complicated involves oil and syringes and stuff.


I think you have a more serious issue than your oil pump, which iirc only works when the engine is running, but don't quote me on that, either way you have no compression at all, consider new barrel and piston, check the bottom end for end float, 30 psi, you may as well blow the piston up and down.
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ather7on
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PostPosted: 15:49 - 04 Jan 2013    Post subject: Reply with quote
pits wrote:
you want around 150psi of compression....120 and consider rebuilding it.

30 psi, in short she is fucked, like proper fucked, I would imagine new top and bottom end, you have had the head off but no mention (as far as I can see) about replacing the head gasket with the correct size, there are 3 gaskets depending on compression ratio the bike itself has, very complicated involves oil and syringes and stuff.


I think you have a more serious issue than your oil pump, which iirc only works when the engine is running, but don't quote me on that, either way you have no compression at all, consider new barrel and piston, check the bottom end for end float, 30 psi, you may as well blow the piston up and down.
we hav changed the base gasket, cylinder head cover, cylinder head cover seal, cylinder head seal and spark plug seal, like i said iv measured the piston, rings and cylinder bore and they seem ok compared to the book settings, just tried starting the bike and it seems very flat, wont start, but oil pump seems ok (it comes along the pipe a little when trying to start it), also coolant seems to be disappearing???? topped up and topped up but always seems empty with no mark left on cardboard under bike? ideas? im not very good with bikes (obviously!) thanks.
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P.
Red Rocket



Joined: 14 Feb 2008
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PostPosted: 15:51 - 04 Jan 2013    Post subject: Reply with quote
pits wrote:
you want around 150psi of compression....120 and consider rebuilding it.


Mine fully rebuilt from new was 130psi.

Check your battery is fully charged, put some 2 stroke oil down the bore and try compression test again.
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pits
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Joined: 22 Apr 2008
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PostPosted: 17:27 - 04 Jan 2013    Post subject: Reply with quote
ather7on wrote:
pits wrote:
you want around 150psi of compression....120 and consider rebuilding it.

30 psi, in short she is fucked, like proper fucked, I would imagine new top and bottom end, you have had the head off but no mention (as far as I can see) about replacing the head gasket with the correct size, there are 3 gaskets depending on compression ratio the bike itself has, very complicated involves oil and syringes and stuff.


I think you have a more serious issue than your oil pump, which iirc only works when the engine is running, but don't quote me on that, either way you have no compression at all, consider new barrel and piston, check the bottom end for end float, 30 psi, you may as well blow the piston up and down.
we hav changed the base gasket, cylinder head cover, cylinder head cover seal, cylinder head seal and spark plug seal, like i said iv measured the piston, rings and cylinder bore and they seem ok compared to the book settings, just tried starting the bike and it seems very flat, wont start, but oil pump seems ok (it comes along the pipe a little when trying to start it), also coolant seems to be disappearing???? topped up and topped up but always seems empty with no mark left on cardboard under bike? ideas? im not very good with bikes (obviously!) thanks.

Did you measure the compression ratio and fit the correct gasket?
There are 3 gaskets, depending on compression ratio you need to find out what gasket is to be used.
0.3m 0.5m and 0.8m and a very complex way of measuring it, you need to put the piston TDC and measure exactly with a syringe how much two stroke oil takes up the space between the head and piston.

Then there is some maths, which works out the compression ratio, which then tells you what gasket you need to use.

I would go one of three ways here
1. She's fucked
2. She has a cracked blocked
3. You have used the wrong gasket and basically it has no compression
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