Loss of Compression???

6 replies to this topic
  • XXX_Thumper

Posted June 20, 2004 - 06:18 AM


Comparing my 450 to my biddies, it feels like I have quite a bit less compression than him. My bike has more hours than his and it has been all MX. Would out of adjustment valves cause a loss in compression? Just trying to decide how much I need to tear into her. I'm out for 4 more weeks due to two broken fingers. :thumbsup:

  • Satch0922

Posted June 20, 2004 - 09:53 AM


Check the valves first. If one of the exhaust valves isnt seating properly you will loose compression.

  • archangelcycle__

Posted June 20, 2004 - 01:39 PM


when checking the valves you will need to measure face,margin thickness as well as measuring the seat in the head. if those are out of spec take appropriate action by cutting the valve seats and getting new valves. it also could be as simple as the valve springs, check the free length of the springs before doing all the other stuff.

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  • gonzo

Posted June 20, 2004 - 03:59 PM


Have you looked at your jetting yet? You were talking about that before. Make sure that's good first.

  • XXX_Thumper

Posted June 20, 2004 - 04:27 PM


She is all torn apart. I'm going to check the jets out next weekend. I'm traveling all week. Noticed less compression than EX2STROKER's bike so I figure while I'm healing I'll check the valves.

  • gonzo

Posted June 21, 2004 - 12:28 AM


Not a bad idea. I am redoing my top end this week too. I have over 200 hours on the bike and I think it's time.

  • grayracer513

Posted June 21, 2004 - 03:23 PM


First, you have to understand that in a properly working 450F, you don't get to feel much compression with the kickstarter, because it gets released automatically. What feels like compression is actually the piston starting down on the power stroke and trying to pull the valves open. I'm not sure, but I imagine that getting an accurate compression test would require locking the decomp flyweight in an disabled position.

Even so, if this "hard spot" gets hard to find, or feels like "low compression", the most probable reason is that the valves have lost clearance. This is especially true if it feels like it has more compression when it's cold than when it's hot.

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