I actually raised the issue on here a hundred years ago, or so it seems now, and was given this link http://www.seight.com/ignition.html that explained it quite neatly. I also posted it earlier this thread, but the jist of it is:sidecar wrote:
The dizzys that you've looked at could easily be different to the ones that I've worked on and therefore may have had less mechanical advance.
Cheers,
Pete
The standard SD1 distributor has bob weights that are supposed to give 26 degrees of mechanical advance but they only give 22 degrees. The 3.5 litre engine needs 34 degrees of total timing at wide open throttle and this should be all in by 3000-3500rpm. The standard advance springs do not allow maximum advance until about 4000-5000rpm. You can replace the springs in the distributor with weaker ones (available from Real Steel among others) to improve this. Using this timing will result in about 12 degrees at idle which works very well with a fast road cam. It is enough to get the idle nice and smooth but not so much as to make starting difficult.
I believe the stock setting for static, according to the Rover book of words, is about 6 degrees at low idle, which, when you add the 22 degrees mech advance, gives naff all near what you really want. Obviously, if you start with 12 degrees, you are doing a lot better, but it all seems very strange to me that Rover would make what appears to be a mass production boo boo in the potential power stakes.
I actually found advancing the static much more than Rover quote made starting harder. it would churn for ages before catching some days.
I fitted the springs and ground some steel off, and I now get a full 36 degrees advance at about 3000rpm plus a nice low advance for start-up.
