Wed Jan 10, 2018 4:04 pm by Monsonmotors
To do it right you have to check that mechanical and vacuum advances are working. They are often both inop.
Do this: with vacuum advance plugged off install a timing light and rev engine a bit. Timing should always go back to the same place and mechanical advance should be all in by 2000 RPM or so. Expect 15-17 degrees added by mechanical advance. With engine off remove distributor cap, grab rotor, twist in direction of rotation and release. It should snap back. A rotor that is lazy to return means that mechanical advance is sketchy at best. It needs to be lubricated and exercised until it's free again. VERY common to see stuck mechanical advance. NO movement of rotor at all.
With cap back on start engine and apply vacuum to vacuum advance. Again you should see 15ish degrees added (this time at idle). If not, vacuum advance is inop. Very common.
Ignition advance is to make up for amount of time that combustion takes at any load or engine speed.
Base timing (where dist. is set) plus mechanical advance plus vacuum advance equals total advance.
An old Motors Manual has all these specs in the distributor section.
Setting a vehicle's ignition base timing ASSUMES that mechanical and vacuum advances are working. MANY a distributor has been mistakenly clocked in the over-advanced position to make up for poor engine performance. Everyone seems to blame ignition timing for all performance woes. Check to see if all your advances are working.
Point dwell or point gap is directly related to timing.