"Dave Plowman (News)" wrote in message
...
In article ,
Dave Baker wrote:

"Dave Plowman (News)" wrote in message
...
The posts from Dave Baker and Roger Mills mirror exactly the discussion
on
the other group. ;-)

If the other group has numpties like me on it then not surprising.

My position is I'm happy that moving the plate the points are on via the
vacuum unit does effect the phasing - but I'm still not sure about the
centrifugal advance. Thing is some dizzies without a vacuum advance still
have a rotor arm with a hockey stick shaped contact.

The discussion came about when details were sought about locking a
distributor so it is only a trigger and HT switch when using a mapped
electronic ignition. Do you set the phasing on full retard, half way, or
full advance?

There's no real difference in where you want the rotor arm with mapped
ignition. It needs to be far enough alongside each cap contact at minimal
ignition advance so that it can advance by another 30 degrees or whatever
and still be adjacent. Given that's 15 degrees of rotor arm movement you can
work it all out by measuring the length of the rotor arm contact. In terms
of direction of rotation you therefore want the rotor arm to be well
alongside the cap contact i.e. towards the back end of it when the crank is
at TDC or just before that.

Easiest way is to check that with the crank anywhere between 5 degrees BTDC
and 40 degrees BTDC the rotor arm is still adjacent to its cap contact. Even
at cranking speed you're never going to spark after TDC, probably 5 to 10
degrees BTDC whether with mapped ignition or points and max advance is
unlikely to be more than 40 degrees of combined vacuum and centrifugal
advance.

What you can do with mapped which you can't with dizzy ignition is run lots
of low rpm timing. Most engines actually want about 20 degrees of advance
even at tickover but if you put that into a dizzy system you can't then add
enough centrifugal advance for higher rpm and there'll be too much advance
for starting purposes. The engine will try to kick back when it's cranking
over. With mapped you can crank at say 8 degrees advance and then go
straight to 20 at idle speed. In fact you can set optimum idle advance just
by getting the highest idle speed as you alter the advance with the throttle
shut then adjust the throttle stop to pull the idle rpm back.
--
Dave Baker