Question About Auto Advance

Discussion in 'Vintage Classics' started by James E Preuss, Apr 29, 2021.

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  1. James E Preuss

    Jul 10, 2019
    34
    13
    Kirkwood, MO
    My 1971 TR6R Tiger is very hard to start and we have diagnosed several issues related to fuel that have all been resolved. But spark remains intermitant so we have examined the contact points advance to discover that the shaft that opens and closes both sets of points appears loose. (See item 18 on the attached parts manual page.)
    Shouldn't that shaft be fastened tightly to the auto advance plate? If so, how is it secured?
    TIA.

    IgnitionParts.jpg
     
  2. speedrattle

    speedrattle Senior Member

    Feb 19, 2021
    1,113
    243
    appalachia usa
    do you mean the cam?


    its free to rotate against spring tension as the bobweights swing out. thats howthe spark is advanced. the cam should not be so loose that you can wiggle it up down orleft right.

    if its got excessive play from wear yourspark will be erratic.

    if you mean the plate 18 which locates the bobweights and springs , it is a taper fit into the exhaust camshaft, secured with a bolt. it should not be loose.
     
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  3. James E Preuss

    Jul 10, 2019
    34
    13
    Kirkwood, MO
    Speedrattle,
    Thanks for the response. Yes, the cam shaft does move in and out and side to side a bit. Is there a procedure to secure it to the advance plate?
    Thanks,
    Jim
     
  4. speedrattle

    speedrattle Senior Member

    Feb 19, 2021
    1,113
    243
    appalachia usa
    #4 speedrattle, Apr 30, 2021
    Last edited: May 4, 2021
    okay. terminology break so we can understand exsctly what is going on

    sorry for the typoes. my phone is too small for accuracy

    the assembly numbered 18 is tbe AAU, theauto advance unit. it consists of an advance plate, two bobweights, two springs, and tbe AAU points cam.

    the advance plate has a hollow mounting cone in back, tapered to fit into theexhaust camshaft hidden inside the engine. the exhaustcamshaft of the engine is drilled, tapered, and tapped. you push the AAU tapered cone into the end of tbe exhaust camshaft, insert a bolt through tbe assembly into the exhaustcamshaft. tightening thebolt fixes tbeAAU advance plate tightlyin place. it should not move. if it loose, or wobbles when the motor is turned, somethkng is wrong.

    the AAU points cam is tbe only part that should movewith the engine stopped. itisheld in place on the advance plate by the bolt and washer, and in correct rotational position by the springs, centered around the hole where the bolt goes through.

    the points cam is eccentric, and as it rotates with the engine it opens and closes the points, mounted to the points backing plate, no. 20, a completely separate assembly from the AAU, that sits in front and hides all parts of the AAU except the points cam.

    the points cam on a triumph can rotate clockwise 12 degrees as the engine speeds up, then when the engine slows down the springs pull it back counterclockwise.

    the points cam must be free to rotate 12 degrees or the ignition timing cannot advance with rpm, which is required. it has to be loose enough to turn against spring tension, but the bolt shiuld prevent it from moving in and out axially. if there is appreciable wear that allows the points cam to move radially, it needs to be replaced.

    im trying to figure out if the assembly is worn out, which is common, or whether it is behavibg normally and something else is wrong, which is also common.

    which exact parts are moving, and in which directions? all of the problems can be fixed.

    if the points cam is loose and rattly, it will never run right and the points will be impossible to gap. in that case, an inexpensive electronic ignition kit will return the system to better than original

    sorry for the length. just trying to figure out what is goibg on
     
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  5. James E Preuss

    Jul 10, 2019
    34
    13
    Kirkwood, MO
    Great information. The cam shaft is moving axially on the AAU
    plate but is also loose on the plate allowing it to wobble. We'll take a look at that and hope it can be fixed since the AAU unit, #18, is rare and expensive (over $100 for a NOS lucas part.)
    Thanks.
     
  6. speedrattle

    speedrattle Senior Member

    Feb 19, 2021
    1,113
    243
    appalachia usa
    #6 speedrattle, Apr 30, 2021
    Last edited: May 4, 2021
    rare is correct, and swap meets are full of worn-out ones. an electronic ignition kit costs only a little more, and will likely solve your ignition problems permanently, with no maintenance.

    the points cam is centered on the advance plate by a central hollow tube, the one the bolt goes through. the inside of the cam and the outside of the tube will wear, giving radial play.

    the points cam moving in and out a couple of millimeters is fine, if the advance plate behind it stays still. the points cam has to be loose enough not to bind up, and its movement is limited by the bolt and washer, part 10.

    if the entire advance plate and AAU assembly is moving axially in and out, while stll firmly bolted into the end of the hidden exhaust camshaft, then the camshaft bushing thrust surface inside the engine is worn thin. it is much more likely that the bolt holding the AAU advance plate to the exhaust camshaft is loose, or a too-long bolt is there, bottomed out inside the end of the exhaust camshaft.

    if you havent done it, undo the two wires to the points and take off the points plate entirely so you can look behind it. it maybe that the entire AAU assembly is loose and just needs to be reattached correctly.
     
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