Valve check issues

Discussion in 'Crazy-Awesome almost Dakar racers (950/990cc)' started by Yascher, May 5, 2013.

  1. Yascher

    Yascher Been here awhile

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    I'm doing a valve adjustment now on the bike and having trouble to catch the real gap.
    Initially, after bike been ridden and cooled down for a day I checked the gaps and these were almost good, except for a couple bit short.
    After pulling the shims out for measurement and putting everything back together and remeasuring the gaps these became much tighter, the more I was trying to cycle the engine to seat the things properly, the tighter it was becoming....however I do not believe that cycling it would effect the gap and push oil out from between valve stem, shim and bucket and so on....anyway the new gaps are still much tighter than before disassembly. I'm experienced enough in this field and have not seen this happening before, all shims and buckets were taken out only as single pairs and there is no way these could be misplaced and so on. Additionally all shim pair were exactly same thickness from factory.
    Checked the gaps at different lobe position of the cams, but same results

    Any ideas or similar experience with 990?
    #1
  2. v8toilet

    v8toilet Nothing to see here.

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    Sillly question as you are saying your experienced but you did remember to install smaller shims to gain more clearance as opposed to installing larger ones? Sometimes for whatever reason folks get their wires crossed here, they want the gap bigger but somehow that translates in to thinking the shims should be bigger.

    Also you should check the clearances with the cams at the specified locations.

    Link here:

    http://paochow.com/forum/index.php?topic=45.0
    #2
  3. bigkuri

    bigkuri Old-timer

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    They do change a little. Are you cycling the engine anticlockwise? Not going past marks even by a little? Using a decent digital micrometre?
    I generally measure once, cycle engine round, measure again. Write it all down. Reshim it, measure. I've found after reshim it changes to max of 0.05 from what it should be as calculated.
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  4. Yascher

    Yascher Been here awhile

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    I did not change shims, I used the original ones in original positions and 2nd, 3, 4, 5 ....measures after reassembly were getting tighter after cycling the engine anticlockwise than the first measure before removing the shims for measurement.

    Until I can get a sure measurement of the gap, there is no point to get new shims there.

    On all my other twin engines gap stays mostly constant after engine cycling, here it's floating and is becoming tighter....which is strange
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  5. Yascher

    Yascher Been here awhile

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    I've read all this and it is standard procedure for any twin.
    Ive checked clearance in all unloaded positions, it stays rather constant on the cam circumference, as it should be
    #5
  6. charlie264

    charlie264 Long timer

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    Installed, mixed around the pair of buckets?
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  7. bigkuri

    bigkuri Old-timer

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    Ah, sorry I misread. Original shims, and bucket/shim the same to each valve, then you must be:
    - turning the engine clockwise, even the smallest amount?
    - not measured correctly the first time around. Maybe you were getting the feel of the slip of the gauge, or some oil under the bucket, etc..

    Measure twice, rotating a cycle between, write everything down, and put it down to something unknown.
    #7
  8. Yascher

    Yascher Been here awhile

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    Got a set of HD shims, playing with these until I get a proper final gap, still experiencing problem with getting the correct measure, probably it has something to do with difficult entry angle at least to the rear ex valves, which leads to some sticking of the feeler gauges, however the other measurement are not completely accurate, as calculated new shim sometimes gives a real overshoot, which means initial gap measurement was not correct!!! Question why it is not correct and has changed a lot after first measurement before removing the original shims and after reinstalling these back??? I've fully cleaned the mating areas and lubed everything, so no foreign material is getting in between and altering measurements.
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  9. Yascher

    Yascher Been here awhile

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    What difference engine rotation has on the valve clearance???
    Same for exact cam position? As far as it in unloaded position, circumference should be of the same diameter, unless you have a bent cam
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  10. Yascher

    Yascher Been here awhile

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    nope
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  11. charlie264

    charlie264 Long timer

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    Strange. All you can do is re-shim to get the right gap..Button it up and do a few miles and recheck.

    What torque setting are you using for the cam bridges, perhaps whoever put them on before you didn't torque them up correctly.
    #11
  12. MAXVERT

    MAXVERT OG on da OC

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    For checking the rear cylinders clearance I like to put a slight bend,
    radius in the feeler gauges I'm using.
    They should slide right in from the rear :D

    Max
    #12
  13. bigkuri

    bigkuri Old-timer

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    Hi, I think that you know this, as you have great sounding mechanical knowledge! :)
    It is all about cam position, and as such the engine rotation is important simply to ensure the cams are in the same position. In the past I have had different gap readings from a tight cam-chain - it was slightly different on separate rotations as the cam chain had tight spots. It's the same reason that ANY clockwise movement when you are lining up the marks is bad - as a clockwise movement is unloading the cam chain, which may or may not move the lobes.

    In terms of exact cam lobe position - well I just figure it is not a constant radius. Maybe the flat side is a constant radius, but better safe than sorry.
    #13
  14. roookie1

    roookie1 Been here awhile

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    For that to be possible you have to have play in Your camholder thingy , I hope not, that means your top ends are toast..Set the cams on the mark (dot, X) thats the point of measuring, for me at least..
    #14