Correct compression for Yamaha DT80

Discussion in 'Motorbike Technical Discussion' started by Masospaghetti, May 25, 2005.

  1. Did a compression check on my bike today - showed about 110 PSI and held
    steady. Is this too low? My haynes manual doesn't say what allowable
    compression is.

    Masospaghetti, May 25, 2005
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  2. Was the engine warmed up? Did you hold the throttle open so it could
    get air to compress?

    I would consider anything less than about 130 psi to be the lower limit
    before I would do a top end. But you just re-ringed the engine didn't
    Are the rings broken in yet?
    krusty kritter, May 25, 2005
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  3. Two-strokes will always record lower psi than four-trokes, won't they?

    130psi sounds fine to me.
    The Older Gentleman, May 25, 2005
  4. I wouldn't say "always", but I would say that cranking compression is
    *typically* lower. Firing every revolution instead of every other
    revolution makes 2-smokes torquey with less BMEP...
    krusty kritter, May 25, 2005
  5. The engine had been sitting for about a half hour when I did the test. I
    didn't open the throttle but I gave it a few good kicks.

    Yeah, it just has new rings. How long until they break in?
    Masospaghetti, May 26, 2005
  6. The normal compression test procedure is warm engine, full throttle.
    The compression should come up smoothly to a maximum reading over about
    5 turns of the engine. This is because the compression tester has a bit
    of extra air volume inside of it and that reduces the cranking
    compression of the engine. The compression tester will have a tire
    valve built into it so it traps the small blasts of air it gets on each
    compression stroke...
    It takes a few hundred miles for steel rings to break in, but only
    about 20 minutes for cast iron rings to break in. Cast iron rings run
    in smooth shiny chrome plated clyinder bores, steel rings run in
    rough-looking iron bores that will usually still have a cross-hatched
    pattern of scratches from the cylinder hone...
    krusty kritter, May 26, 2005
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