No vacuum from vacuum advance

[buddy84GMC]

I have an '84 GMC Sierra with a '72 small block 400 in it with HEI.  Anyways, I wanted to check the timing with the light since I heard the valves pinging and I retarded the timing to 4 degrees advanced from I think it was 8.  The truck runs a little slugier, but I guess that what happens when you retard timing and want to run 87 in it. 

My question is that I had no vacuum at idle coming from the carb for the vac. advance.  I did get vacuum when I revved it, but I thought I had to unplug the vac. advance to check the timing and this is when I noticed no suction.  I have 2 ports for vacuum off the carb, but the other is for something else...  Is it normal to not have very much suction from the vac advance when in idle?  Thanks a lot. 

The truck runs great now with no hesitation or pinging, but I was curious about that suction. 

[VileZambonie]

Hook the vacuum advance to manifold vacuum rather than ported vacuum and adjust your idle speed.

Valves don't ping. Pinging is the sound you are hearing from either preignition or detonation or both. If you have to excessively retard the timing to make it run on pump gas, make sure you are using the correct spark plug heat range and maybe it's time for a good decarbonizing or some top engine cleaner.

[Fast68Chevy]

each engine and each distributor is different and unique

you will have to set the timing and use whichever vacuum source for the vacuum advance as needed to set the engine timing correctly

sometimes i have an engine that runs correctly without vacuum advance at all

each is different, i have found

i use a vacuum guage to set the timing usually

without vacuum advance connected

and then i go from there after restarting it and making sure that it is not too far advanced initially(slow cranking speed is a giveaway of this problem)


good luck