Powerdynamo, no advance controil on 2 strokes which had a centrifugal govenor

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My (2 stroke) motorcycle had a centrifugal govenor for advance control. The new Powerdynamo system now says it has fixed timing. How should this work on my bike?

Till about the mid 60s most 2 stroke engines had a centrifugal govenor that controlled ignition timing. It permitted those engines to start near TDC (top dead center) and as soon as the engine run, changed to driving position of about 28 degrees BTDC (before top dead center

This was mainly needed because of the bad carburants at the time, having a mere 60-70 octan (before the war even as low as 40 octan). Starting with full normal advance was extremely difficult with those carburants. Sa a centrifugal govenor was used to help starting.

With carburants getting much better in the 70s, this expensive and difficult to maintain part was supressed. You will not find it any more on later 2 strokes (sure you find it on 4 strokes!). 

TDC - the highest point a piston can reach

the normal moment of ignition is always a few millimeters before this. therefore called BTDC (before top dead center).

Old 2 strokes work with something like 2 to 8mm BTDC, depending on make and the older the more advance was needed.


With the modern carburants (which now well into the 90s are actually to hot for those old motorcycles, but you have no choice but to use it) the need to change timing during starting is no longer there. Even less so with the high power condenser discharge ignition (CDI) as you have with Powerdynamo.

You even should get a situation where the old factory value for normal ignition might no longer be valid. Flame development is so rapid that you need to set ignition nearer to TDC than your old manual might indicate. This we experience in most motorcycles made before about 1970 and now equipped with Powerdynamo.

Sure, a Powerdynamo system can be equipped with advance control as we e.g. do as a rule with the MZ BK flat twin which has a tendency for violent kickbacks otherwise.
But in most cases this is a waste of money.


 



There is however some sort of advance/retard inherent in the material.

The diagrams here shows timing of spark in depencence of engine speed in our internal sensor systems 

  • diagram above with 112mm rotor, 
  • below with 103mm rotor).



    The line zero respresents the angle you set at point of installation by aligning stator and rotor markings with the piston in the wanted ignition position (say 2 mm before TDC which might be something like 28 or so).

    At start, something below 1200 revs ignition will happen later, nearer to TDC, here shown in the negative area of the graph. During main driving revs, say between  1200 and 4000 ignition happens in about the set timing (zero line) and in higher revs it again drops back a little towards TDC (the negative area here). 

    This is something inherent in the material, but corresponds very well to what a 2-stroke engine would like.

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