73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: fiddler on July 12, 2020, 06:05:32 pm
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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...
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Here is a pic of mine.
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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!!
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Fiddler, can you scrub the indicator with a small wire brush? Post a pic.
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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.
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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....
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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.
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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.