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Ignition timing

  • 10 Aug 2019 7:04 AM
    Message # 7821638

    I have some difficulty knowing, at what point the spark plug ignites the fuel air mix in the front cylinder.  With igition timing, I know the narrow lobe is for the front cylinder, and the wide lobe, is for the rear cylinder.  

    When the rear exhaust valve is fully closed, at that point, where the exhaust tappet just starts to loosen, I’m led to believe, that the front piston, is 3/8 “ before TDC.  This tecnique of course, is for heads still on.

    Relative to front piston stroke stroke,  what I want to know is, when should the contact breaker  actually start to open and spark.  Is it also adjusted, so it just starts to open and spark at 3/8 “ BTDC on the front cylinder ?  

    On my 1934 45” Standard Scout, it is coil ignition.  There is no magneto.  The contact breaker point assembly, is mounted on the right hand side of the timing case, in between the rear cylinder header and the return oil pump.  

    So, the question now is, do I set the points, so they just start to open (spark) at that point, where the front cylinder, is 3/8 “ Before Top Dead Centre ?  ...and do I set it at this, with the right twist grip in the retard position, or the advanced position ?  Which one ?  Any assistance is greatly appreciated.  Thanks in advance.

    Last modified: 10 Aug 2019 4:27 PM | Danny Marks
  • 17 Aug 2019 8:54 PM
    Reply # 7833992 on 7821638

    Ignition is always set at the maximum advance.

    It is a bit dangerous to use the cam lobe as the index point of ignition timing, partly because the exact moment of valve movement depends on the valve lash and partly because of the wear and play of the cam lobe and mechanism. That is if you haven't measured the exact time the valve closes/opens with a particular valve lash when the heads are off...and written a note in the engine logbook of it...

    Now...it is also a bit dangerous to assume that the ignition cam opens the breaker at the exact time that it is supposed to, on both cylinders, because the breaker cam also wears and wears uneven...That should also be measured with the correct breaker gap setting, on both cylinders with the heads off...and noted in the logbook...the other cylinder can ignite way late if the cam is worn or if it is a cam for a engine with 45 degree cylinder vee split..

    With iron heads, it is better to be on the safe side and ignite a bit later than early, there is a lot of different ignition timings in the service books floating around, everything between 7/32 for th 37ci, to 5/8 special racing.

    Standard 101 37ci engine with iron heads is 5,5mm 7/32"

    Standard 101 45ci engine with iron heads is 7,9mm 5/16"

    Sport Scout, 3/8"-7/16" in one of the books. There is no specific notes and  without knowing, I suppose aluminium heads 11,1mm 7/16" and Iron heads 9,5mm 3/8", but I would/might start with 5/16" on a fresh engine, not yet run in..

    I tried 7/16" on my engine (by mistake) with iron heads and the pistons nipped.

    So,what to do with the heads on? A rough setting can be done with a semi stiff bailing wire. If you can see down the spark plug hole and measure what distance below the cylinder top surface and the piston top at TDC, (maybe already noted in the logbook?) then take a bailing wire and bend an L on it, the L foot length should be  5/16" (or ign. time of your choice) + the distance to the piston. Put it through the spark plug hole, rest the wire on the cylinder top surface at the edge and when the piston touches the wire tip, then the breaker should just part, a paper between the points should loosen up..  

    Last modified: 17 Aug 2019 10:33 PM | Carl-Erik Renquist
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