• This topic has 5 replies, 3 voices, and was last updated 1 week ago by Brian.
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    • #15392
      Brian
      Member
      • Topics: 16
      • Replies: 33
      @woody
      Uk
      Gt125r

      Just rebuilt the front head, new gaskets, lapped valves, used piston, new rings, and still low compression. 100psi. Checked valve clearances, intake reads 0.08 0.05 exhaust 0.10 0.10. Could this cause the compression issues? Thanks in advance

    • #15393
      Brian
      Member
      • Topics: 16
      • Replies: 33
      @woody
      Uk
      Gt125r

      Forgot to mention, added a little oil in cylinder and only went up 25psi so rings are ok. Just need broke in.

    • #15394
      Simon
      Member
      • Topics: 1
      • Replies: 273
      @syco
      Kent
      Gt250 f1

      The intakes should be between 0.1mm-0.2mm exhaust 0.2mm-0.3mm when cold. To tigh clearance will open the valves to soon & also as it warms up likely to keep the valve slightly open.

    • #15395
      Brian
      Member
      • Topics: 16
      • Replies: 33
      @woody
      Uk
      Gt125r

      Thank you

    • #15426
      LaserBeam
      Member
      • Topics: 4
      • Replies: 49
      @laserbeam
      Earth
      Hyosung GT250R 2008

      Just rebuilt the front head, new gaskets, lapped valves, used piston, new rings, and still low compression. 100psi. Checked valve clearances, intake reads 0.08 0.05 exhaust 0.10 0.10. Could this cause the compression issues? Thanks in advance

      Did you put new rings on old pistons, then pop them back in to old cylinders?

      What did your cylinder walls look like? Do they have deep, vertical scratches?

      Did the old pistons have scratches that match the scratches on the cyclinder walls?

      Rings are important & relatively easy to swap. Honing the Cylinder walls is pretty darn important to but it’s messy & expensive if you get a pro to do it properly.

      I jrecently did it myself but didn’t do my valves yet, Perhaps you could help me with that?

      I’ll be doing a photo tutorial of my D.I.Y. rings, pistons & cylinder honing.

      My guess is that you’ve got a lack of compression  from slightly worn pistons & cylinder walls. It is possible that your timing is out a bit – that makes it seem like low compression when ridiing. Your exhaust valve gaps should be 0.2 – 0.3 for a 250 i think. What bike is it? Bigger shims in there should sort that out. The Intakle Clearances should be 0,1 i think, so you could add some shims to those to open them up a bit more too. As it is i dont think your bike is getting enough Fuel/Air in or exhaust out. It’s not bad but you may notice better power with more open valves, if it is a 250.

      PS. I’d buy 2x new pistons, with rings, pins, retaining clips & both head gaskets. Hyosung have them but aftermarket piston kits will save you a few dollars. I highly recomend the Original Equipment Hyosung GASKETS – they are like rubber, not paper. I used a paper gasket on the head and an orignal metal gasket above it. The paper is ok but for stator cover & clutch cover gaskets, the hyosung ones stop oil leaks way better.

       

      -----LaserBeam---->

    • #15430
      Brian
      Member
      • Topics: 16
      • Replies: 33
      @woody
      Uk
      Gt125r

      Ye I honed the cylinder before rebuild and the walls where fine. Timing is spot on, only thing that needs doing is valve clearances, if that doesn’t work new pistons and rings it is

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