Automatic Advance not working?

  • Thread starter Thread starter laverlesmains
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laverlesmains

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Hi

On my 1975 3C, I finally got around to hooking up the strobe light to check ignition timing.  It appears that the ignition doesn't advance as I rev the bike up to the 4000 range..  I didn't think that the bike would rev if the ignition wasn't advancing, but reading through some old threads here it appears that the bike can seem to run fine even if the ignition isn't advancing.. 

is this true? 

Cheers, Tim
 
Tim,

The ignition has a very simple advance - it does it only once and lobs the entire advance in at around 2000rpm (at least, I think it's 2000rpm). After that, it doesn't change....

HTH

Pete
 
It's possible to move the advance point by changing the gap between the trigger and the sensor.
Close the gap (how?) and the advance happens earlier. Open it up (file away) and it happens later. Or is it the other way around.
You should always try to ensure an even gap between trigger and both sensors (3 on a 120) so the advance happens at the same time, otherwise you will have 2 (3 on a 120) different advance curves.
 
you mean advance steps Grant, the original system has no curve.

Tim hook the stobe up to the centre pot and see if you have an advance step at around 2000/2500 revs,
if you have then reconnect to one of the outers (RH is easist) and check again, if the advance does not happen, then likely as not the polarity of the pickup is round the wrong way (easilly changed at the chock block under the rear of the fuel tank, or one of the wires under the generator rotor is worn through from rubbing/roation and grounding when it shouldnt, in either case if this your problem the piston damage will occur, so dont ride until you are sorted.It is possible that both pickups are round the wrong way or wires damaged

CLEM

ps this assumes you have original Bosch BTZ (two coils under front of tank, boxes left and right under side panels) more likely on a 3C, Bosch CDI ignition (one box and mini coils on a unit under the tank front right)
 
Close gap on pickups and full advance comes in sooner, open up and it comes in later. It's all or nothing. Check that gap.
Ciao
Paul
 
CLEM et al

Hooked up strobe to center and no advance.  No advance on any cylinder.  It is the original ignition system.

Cheers, Tim
 
Tim, as I mentioned, the CDI boxes do fail over a period of decades. I tried to remove the circuit board from one of my dead ones but it was totally encased in epoxy resin, and I was unable to clear off the board to see what the circuits were.
It seems improbable that the box would still able to charge up capacitors and discharge them on trigger but have the advance circuit disabled, but this appears to be what's happened here. You may well be able to ride it but performance is going to be bloody awful with no advance at all, and poor starting will probably finish off the starter clutch. There is a solution; buy one of the available aftermarket ignitions and have no more problems for the remaining life of you and the bike ;). And if you get the Redax one the battery will actually be able to charge up and you won't have to push start it all the time, plus you can add some accessories (heated grips, LED light strips, CB radio :D :D).

Ken
 
Right.  that settles it.    As soon as I can secure a puller it's out with the old and in with the DMC.

Thanks for all the replies

Cheers, Tim
 
Lads,  have repaired more than a  few HKZ in the past,  you cut the large side face off ( metal bladed hacksaw ) and dig downwards towards the PCB,  there are 3 SCR's.  Two SCR triggers for the 3 coils and one that forms part of the shunt regulator for the exciter coil on the stator plate.  Clamps the energy transfer capy at 400VDC or so. 

    Not that hard actually to repair,  not a lot on the board.  The advance is based on the symmetric output pulse of the impulse cam to reluctor pickup.  As the impulse cam lobe approaches the reluctor a positive  going pulse is produced,  this fires  at normal motor rpm, as the impulse cam lobe leaves the reluctor pole face a negative pulse is produced (*).  When the crank rpm rises so does the rate of magnetic flux change and the voltage level of these two pulses rises  quickly to over 80V,  if you leave them O/C, naughty!  At a fixed voltage set point,  the actual SCR trigger signal is switched from trailing to leading edge of the lobe, plus or minus some "natural" advance retard as the set point slides up and down the pulse voltage waveform.  Clear as mud.  Grin.

  (*) Why if you incorrectly reverse the polarity of the VR pickups your advance works backwards...


  As for repairing BTZ,  like they are worth repairing,  broken from new.  All the same they were reliable....  HTH John.
 
John,

If I am understanding what you say correctly (which is probably unlikely  ::) ) is what you are saying the advance is an inherent part of the way the HKZ works? and if the HKZ is working the advance must be too?  Does it mean the only way you wouldn't get advance is if the VR pickup polarity has been accidentally reversed?

If so, that would mean all Tim has to do is reverse his pickups?

Alternatively, I didn't understand anything  :D :D

Pete
 
Pete,  you had the correct concept of failed trigger switch from trailing to leading edge with your first post.  Happens more common on BTZ than HKZ.
Easy things first for OP,  and as others suggest,  inspect the leadout wires for the common failed insulatoin and possible shorting of the output signal voltage.

  Often it is easy after inspection to wire up the HKZ and test with strobe for actual firing point angle on the crank.  And then reverse the VR connection and once more check the actual firing angle point on the crank.

  VR's have a "natural" advance characteristic of some 12 degrees,  which may or may not be used by the control module.  If that 12 degrees is applied from a cam timed motor,  then that gives 24 degrees crank and with a static advance on cold crank of 8 degrees yields the magic "32DBTDC" often found on electronic ignitions for British engines of the era.  Nifty.

HTH John.
 
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