Can someone explain to me the interaction of the vacuum advance and the mechanical advance on the distributor please? When I set my timing using a strobe light, I do it to give 34 degrees advance at 3,000 RPM as by this point the advance should be "all in". This in turn means that the timing at idle falls where it will and that's OK as you are more concerned about getting it right at higher power settings. You time it with the vaccuum hose off the distributor, presumably so that it doesn't affect your readings and only the mechanical advance is producing the 34 degrees advance. Let me know if I'm OK up to this point... So if you now connect the vacuum hose, is the idea that the timing will immediately advance to 7.5 degrees or whatever the figure is mean to be at idle, or is there not enough vac at this point to actually move the plate? Does it only start working when you increase throttle to combat flat spots whilst waiting for the mechanical? And finally...once the mechanical comes in, does it "override" the vac or add to it? For example, if you had your dynamic set at 34 degrees at 3,000 rpm and then connect up the vac, will you still have 34 degrees or will you have added the 7.5 from the vac to give a total advance of 41.5? I now sit back and and wait for somebody to point out the bleeding' obvious that I have missed Thanks!