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Tci unit

Discussion in 'XJ Technical Chat' started by joe elliff, Nov 13, 2018.

  1. joe elliff

    joe elliff Active Member

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    ok. I’ve looked and looked and well. I don’t wanna keep looking so here I am.

    Which tci unit is correct for a xj750r?
    Are their multiple correct tci’s?
    Is there a spot here which lists the different tci units by tci model to the corresponding xj?

    In particular I want to know which tci is correct or which tci’s will work for a 750r

    Thanks. Joe
     
  2. XJ550H

    XJ550H Well-Known Member Premium Member

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  3. XJ550H

    XJ550H Well-Known Member Premium Member

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  4. XJ550H

    XJ550H Well-Known Member Premium Member

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    as a side note there is a black label tci out there . it was the original tci for the XJ650 Midnight special.
     
  5. joe elliff

    joe elliff Active Member

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    Ok. I’m now confused.

    So as I read this the 5g2 ignitor/tci unit is correct for the 650/750 models. Ok. No problem.

    I bought a 750rj with a 4u8. That tci belongs to a 550 then? I just looked that up and

    The 4u8 tci operated flawlessly for well over a year and 1500-2000 miles. I suppose that’s an alternative then

    Interesting to say the least
     
  6. joe elliff

    joe elliff Active Member

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    Old one
     

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  7. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    The 550 TCI will work, but the timing advance will not be ideal for the engine.
     
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  8. Franz

    Franz Well-Known Member

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    The timing advance controlled by the TCI is that the same as the dwell angle that used to be controlled mechanically by centrifugal force? I remember my fathers car with the distributor and points with the springs which were forced away from the distributor shaft as engine revs increased by centrifugal force. That must have been less accurate than the TCI systems we have now.
     
  9. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Dwell is not the same as ignition advance/retard, but yes the TCI replaces the mechanical distributor.

    Dwell is that amount of time that the ignition points are closed. Advance and retard set the time that the ignition fires relative to piston position (degrees of crankshaft rotation before or after top dead center).
     
    Last edited: Nov 14, 2018
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  10. turpentyne

    turpentyne Active Member

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    Mechanical advance underneath the top plate. And then there was the vacuum advance.
     
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  11. Franz

    Franz Well-Known Member

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    That is right forgot about the vacuum advance but I am not 100% sure how it all works.
     
  12. XJ550H

    XJ550H Well-Known Member Premium Member

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    4U8 unit is a 550 wait until you put the correct one in there let us know what you think then.

    we had a member who had a 900 tci (iirc) in his 550 ran like sh@@ untill i sold him one of my spare 550 units
     
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  13. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Vacuum advance works just like mechanical advance, but the engine vacuum pulls on a diaphram that moves the spark advance instead of relying on weights and centrifugal force.
    It also leaves you (well, me anyway) befuddled sometimes when you don't notice that there's a pinhole in the vacuum line.
     
  14. Franz

    Franz Well-Known Member

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    Thanks k-moe so the mechanical advance or vacuum advance makes the spark happen earlier at higher engine speeds so that the engine will produce the correct power output. What is clever is how the designers can work all this out so that decent power is produced throughout the rev range. On some cars now (I am not sure about motorcycles) they have dephasers to alter the cam timing too which operate on engine oil pressure to give a better spread of power.

    On two strokes I remember Yamaha had power valves in the engine to produce a better power spread throughout the rev range. That was the Yamaha Power Valve System on the 500cc engine and the 350 water cooled twins. I assume these altered the size of the ports? Interesting how they work all this out. And my first bike I got for work was an MZ TS150, a cheap and smokey two stroke single.
     
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  15. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    Correct. Yamaha also had (has?) a similar system for the exhaust system on some of their fancier 4-strokes.
     
  16. Franz

    Franz Well-Known Member

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    I did not know that they used them on four strokes too.
    Koenigsegg are designing freevalve camless engines now but I don't know if any motorcycle manufacturer is going to use them.
     
  17. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

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    I feel like I remember reading about Honda playing with pneumatic actuated valves in the late 90's.
     
  18. Franz

    Franz Well-Known Member

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    Well if it is adopted for motorcycles with all of the electronic control systems they have today they should be able to fine tune the fuel air delivery and exhaust emmissions even further. Don't know if Ducati would use them or stick to their Desmodromic system which I read they used to eliminate valve bounce at high RPM on their racing engines. It is all very interesting.
     
  19. Rooster53

    Rooster53 Well-Known Member Premium Member

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  20. Franz

    Franz Well-Known Member

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    That is clever Rooster.
     

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