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How do you know if it's tuned just right?

Discussion in 'XJ Technical Chat' started by e13design, Jul 24, 2017.

  1. e13design

    e13design Member

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    So I've had my seca for almost 10 years. Year down, full restore. Valves adjusted, electrical Gremlins sorted, carbs cleaned (and cleaned and cleaned and cleaned...), Fuel mixture colortuned, and carbs synced.
    This is my first bike (though I've ridden some more modern ones) so I'm not sure how it should sound, feel when it's all just right.
    Most recently I moved to LA and am driving in 105° weather.
    At 70mph the engine was vibrating more than at other speeds so I checked the sync and it was a little off probably from shipping or climate/elevation change.
    So now it barely feels like it's running when I'm cruising at 70mph.
    But... Getting there I now have less acceleration and more vibration at lower speeds.
    I checked sync again and I'm still ok.
    Is that just the payoff for being able to ride at 70mph?
    I'm on the highway 75-80% of the time now so most of my ride is at higher speeds.
    Any thoughts?
     
  2. XJ550H

    XJ550H Well-Known Member Premium Member

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    syncing balances the out put of the cylinders by tuning them to the weakest cylinder
    you are really syncing the cylinders not the carbs,
    so with you syncing them again they are more intune with the weakest cylinder and not fighting each other at 70mph.
    which would affect the lower end quickness because you have decreased your total Horse power.
     
  3. e13design

    e13design Member

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    Ok, that makes sense. So (and I understand this is probably and impossible question) how to I increase the output of my weakest cylinder to increase my total horse power?
    Is this where I get into compression testing?
     
  4. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Cylinder hone/bore and new rings, replace head gasket, etc. i.e. top-end rebuild.

    Compression testing will tell you which cylinder is the weakest.
     
    XJ550H and Toomanybikes like this.
  5. JeffK

    JeffK Well-Known Member

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    Interesting take on it XJ....I thought on the Yammy fours, all were sync'ed to the #3 carb?

    jeff
     
  6. Polock

    Polock Well-Known Member

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    when you sync the engine it's mostly just for idle and very low throttle. your opening and closing the butterflies to make the cylinders have equal output.
    BUT the tiny differences you made at idle are just about meaningless at larger throttle openings. it's a percentage/ratio kind of thing.
    say if you sync at idle and change #1 .005 inches from .009 to .014, that's over 50%, but at half throttle (maybe .5 inches) that .005 inches doesn't really matter.
    {.009 +/- .005 =a lot.}but{ .5 +/- .005=insignificant}
    other parts of the tune have way more effect at that point
     
  7. XJ550H

    XJ550H Well-Known Member Premium Member

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    you sync 3 to 4 then 1 to 2 .then sync left pair to right pair ,so you make 1 pair equal then another pair equal , then equal the pairs to each outer
    not carb 1,2 and 4 to 3. when you sync the pairs you sync to the lowest set.
     
  8. Polock

    Polock Well-Known Member

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    sort of, in a way. say #1 is the weakest, the sync screw opens #1, closes #2, so far so good. sync 3 and 4 but only #4 should move. #3 is controlled by the idle screw. now the sets 1/2, 3/4, 1/2 gets opened to sync with 3/4. now #1 the weakest, is the same as the other ones and #3 is in control of all.
    depends on how you look at it
     
  9. e13design

    e13design Member

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    That makes sense, but if everything is out of whack then even at top speeds it would make a difference I would think.

    So understanding all of that; my original question is more along the lines of how can one tell if it's all set and running properly?
     
  10. Polock

    Polock Well-Known Member

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    put it on a dyno. then after that have three people ride it and i bet you get three different opinions.
     
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  11. XJ550H

    XJ550H Well-Known Member Premium Member

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    by how big of a grin you have when you go WOT through the gears.

    first set the valve shims , second do a compression test
    those 2 will tell the condition of the cylinders. if all cylinders are with in the spec of difference between cylinders , then the sync will be as close as you can get followed by color tune , rinse and repete.
    ignition in spec, plug caps replaced and new plugs , carbs cleaned to standing at the alter spec, .
    clean air filter fresh oil

    go ride and enjoy do the grin test.
    short of a top end rebuild and lots of test equipment and finding an old yamaha repair guy, you have done the best you can
     
  12. Rooster53

    Rooster53 Well-Known Member Premium Member

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    Checklist:

    Valves have been checked and are in spec
    Carbs have been through Church of Clean
    Compression numbers are in spec
    No backfiring on deceleration
    No hanging idle
    No flat spots in the throttle and smiley face when approaching redline
    Plugs look good
    Gas mileage is normal

    And most importantly, it will stop reliably as front and rear brakes and tires have had full attention.
     
    Stumplifter likes this.
  13. Stumplifter

    Stumplifter Well-Known Member

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    ^^^ those previous two.

    The grin/ smiley face means you know that you have your bike performing as it should and you can rely on it to get you there and back.
     
  14. Jetfixer

    Jetfixer Well-Known Member

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    By the seat of your pants if you hit 6K and engine is coming alive ...you know it is right ...oh sorry I was day dreaming about riding . You will know it is right when it starts and idles with no unevenness and it runs with steady throttle with no popping or undue noise from the exhaust .... You can get it takes patience and reading Don't give up.
     
  15. cgutz

    cgutz Well-Known Member

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    When it accelerates from a stop smoothly and effortlessly...like it is "breathing" without restriction.

    Then at about 5-6k the power band hits...be holding tight and leaning forward.
     
  16. e13design

    e13design Member

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    Ok, compression test complete (after a bit of a learning curve).
    All readings are 118-120psi.
    So I guess it's about as good as I can get it.
     

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