<VV> timing changes at idle.

[Mark Durham]

Gents, I've read here about doing something to the distributer vacuum
advance to stop the advance from moving the timing at idle speed but
cannot remember what was done.

Today I set my engine rpm to 600, adjusted the timing with the advance
hose off and plugged, reset the idle, rechecked the timing, then
hooked the hose up, and the engine accelerated as the advance moved
the plate a bit. What can I do to get the advance spring stiff enough
that the timing stays stable at idle and will come in later as I
advance RPM?

Mark Durham