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Low compression 13 May 2022 17:24 #867041

  • gtrain
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Hi all I have a problem with my z1000st I had it bored to 1100 fitted a set of cruise image pistons and rings, now its hard to start doesn't idle well, I've check valve clearances check valve seats, carried out a leak down test,but compression is around 120psi across all four cylinders remove the head again and noticed that at individual  top dead centres the pistons don't go to the top of the cylinder, heres  a picture is this normal?
Any advice would be appreciated.
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Low compression 13 May 2022 19:45 #867050

  • Nebr_Rex
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Most engines run with a positive piston deck height [below the top of the cylinder block].
How thick is your base gasket? Have you considered using no base gasket just sealer?
2002 ZRX1200R
81 GPz1100
79 KZ1000st daily ride
79 KZ1000mk2 prodject
78 KZ650sr
78 KZ650b
81 KZ750e
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77 KZ400/440 cafe project
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www.dotheton.com/forum/index.php?topic=39120.0


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Low compression 14 May 2022 02:24 #867060

  • Wookie58
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To be honest if you have 120psi on a freshly bored cold motor I think your problem probably lies elsewhere. Is the cam timing correct ?
PS: are the ring end gaps equally spaced 120 degrees apart ?

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Low compression 14 May 2022 03:01 #867061

  • Warren3200gt
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You don't say if that 120psi is hot or cold engine or throttle open or closed.
If that's cold then it's not too bad. 
If that's throttle closed it's bloody brilliant. 

Z1000J2 somewhat modified!

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Last edit: by Warren3200gt.

Low compression 14 May 2022 07:43 #867071

  • blipco
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What type of compression tester? The type with a rubber end that you push on? Or a hose type?
If its hose type does it have a Schrader valve on the threaded end?, If not it'll read low as the hose adds volume to the combustion chamber.
"Swim against the current, even a dead fish can go with the flow"-somebody (I forget Who)

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Low compression 14 May 2022 15:50 #867099

  • 750 R1
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Am I the only one that's seeing small bits of alloy around the edge of the piston ?  I hope that was all cleaned out...
The following user(s) said Thank You: Nessism, Street Fighter LTD

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Low compression 14 May 2022 16:58 #867108

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Also not mentioned is how much breakin was done to seat the rings  after  motor was built 

Original owner 78 1000 LTD
Mr Turbo Race Kit, MTC 1075 Turbo pistons by PitStop Performance , Falicon Ultra Lite Super Crank, APE everything. Les Holt @ PDM's Billet Goodies . Frame by Chuck Kurzawa @ Logghe Chassis . Deep sump 5qt oil pan. RIP Bill Hahn

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Low compression 14 May 2022 17:00 #867109

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Am I the only one that's seeing small bits of alloy around the edge of the piston
?  I hope that was all cleaned out...


I saw that too but hoped that was small debris from pulling the head back off  ( Hope , hope , hoping  )
 

Original owner 78 1000 LTD
Mr Turbo Race Kit, MTC 1075 Turbo pistons by PitStop Performance , Falicon Ultra Lite Super Crank, APE everything. Les Holt @ PDM's Billet Goodies . Frame by Chuck Kurzawa @ Logghe Chassis . Deep sump 5qt oil pan. RIP Bill Hahn

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Low compression 14 May 2022 18:02 #867114

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Am I the only one that's seeing small bits of alloy around the edge of the piston
?  I hope that was all cleaned out...


I saw that too but hoped that was small debris from pulling the head back off  ( Hope , hope , hoping  )

 
My guess was gasket material. 
1976 KZ 900 A4 kzrider.com/forum/11-projects/613548-1976-kz-900-a4
1976 KZ 900 B1 LTD
1978 KZ 1000 B2 LTD
1980 KZ 750 E1
Kowledge Speaks, But Wisdom Listens.
Jimi Hendrix.

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Low compression 15 May 2022 00:39 #867118

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Am I the only one that's seeing small bits of alloy around the edge of the piston ?  I hope that was all cleaned out...
Look's like gasket scrapings to me but I have old eye's 

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Low compression 15 May 2022 04:42 #867120

  • hardrockminer
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With stock pistons the perimeter of the piston should just reach the top of the cylinder.  So what you're showing in the photo is not normal, but those are not stock pistons if you bored to 1100.  That said, 120 psi is a good reading, maybe a tad on the low side, but more importantly they are all within 15 psi of each other so it seems that your rebuild is OK.

As far as poor idle is concerned, what changes were made to your fuel and air circuits?
I have several restored bikes along with a 2006 Goldwing with a sidecar. My wife has a 2019 Suzuki DR 650 for on and off road.

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Low compression 15 May 2022 07:34 #867125

  • blipco
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Isn't it true that the piston crown height is lower on an aftermarket piston to allow for planing the head?
"Swim against the current, even a dead fish can go with the flow"-somebody (I forget Who)

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