Timing Tab Picture 1974 C10 350 automatic

[fiddler]

So my timing tab has 4 peaks and 3 valleys.  It is on a 1974 chevy C10 stepside pickup.  Automatic with Air conditioning.  The timing tab is rusted over and illegible.  I have been unable to locate a photo of the timing tab.  I have finally resorted to backing off the timing to what I think is 6 degrees and then letting the vacuum advance take it to 10 at idle.  This results in a no pre-ignition condition coming up the mile long hill to the house, but I lost some acceleration capability.  So would I be better to advance the vacuum advance a bit more (it is adjustable) or the static timing a bit more?  In actuality I don't care so much for the loss of acceleration, but I am thinking I will also lose mpg adjusted like that...

[Rapid Roy]

Here is a pic of mine.

[fiddler]

Thank you.  So is that from a 1974 350?  It looks like maybe mine has lost the lower end peak and possibly the upper end peak.  If I go with that assumption I think I can use this pic.  You are the best!!

[bd]

Fiddler, can you scrub the indicator with a small wire brush?  Post a pic.

[Rapid Roy]

Thank you.  So is that from a 1974 350?  It looks like maybe mine has lost the lower end peak and possibly the upper end peak.  If I go with that assumption I think I can use this pic.  You are the best!!

Yes. 74 stock 350 w/HEI timing set at 12 degrees BTD center with advance connected to manifold vacuum.

[fiddler]

So here is a photo of the type of tab that I have on my truck. This photo does not have visible numbers.  I have wire brushed part of it and am able to discern the 8 and 12 spots so made some assumptions after that.  I would need to remove the power steering pump in order to brush the rest of it.  My thought was to do that when I eventually remove the power steering pump, (a truck this old will require everything be removed I suspect), but I would rather not go through that if I can find someone with a photo showing the integers.  At this point I think I have the initial timing at what I think is 6 and I have adjusted the vacuum advance to where I have good acceleration and yet do not ping pulling a load up the hill to my house with a load on the truck.  I think that is where I want it.  At 8 degrees initial timing I seemed to be unable to get rid of pre-ignition with a load on the truck climbing the hill.... 

[bd]

The deep valley is zero.  The initial peaks on either side of zero in both directions are 4° BTDC, or 4° advance, (CCW) and 4° ATDC, or 4° retarded, (CW).  Then continuing in the CCW direction, each valley and each peak adds an additional 2° advance.  So, 8° advance is two peaks CCW from zero.

[fiddler]

Then I guessed right.  I just could not figure how to make it all work in twos, because the first from the zero valley is 4.  Now it all fits and makes sense.  And I don't have to pull the power steering pump....yet.  Thanks much.  I will copy this and put it in the book.