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Vacuum advance- TOO MUCH VACUUM?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Can Can, Jun 20, 2003.

  1. Can Can

    Can Can Pusher Man Staff Member Super Moderator

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    I recently replaced my vacuum advance because the diaphragm was blown and the truck was suffering from vacuum leak symptoms. Well, the new advance fixed my crappy idle and hard starts but...... /forums/images/graemlins/tongue.gif /forums/images/graemlins/mad.gif

    ......now I'm having pinging issues. I can't drop the timing much more because it's already set pretty low and I don't want to affect what little performance I'm getting out of the tired ol' 305. A buddy of mine recommended installing a "vacuum restrictor"(or something like that) because now I'm getting TOO much vacuum.

    If I'd known I would have had these issues I would have bought the adjustable vacuum advance model..... /forums/images/graemlins/rolleyes.gif Does anybody have a clue what he's talking about? What kind of vacuum readings should I be getting at the distributor anyhow?
     
  2. Confedneck

    Confedneck 3/4 ton status

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    well i know you should have NO vac at idle... if that helps any
     
  3. HarryH3

    HarryH3 1 ton status Author

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    You can't get too much vacuum. However, you can mechanically change the amount of total timing that the distributor will pull. This includes the mechanical advance and the vacuum advance. The addition of those two, along with the base timing setting will determine the total timing. Some vacuum cannisters have an adjustment screw internally that will let you adjust the amount of timing advance that is possible with the cannister. They have a small screw inside the tube where the vacuum hose connects. It's much easier to determine what is going on if you have access to a dial-back timing light and an accurate tachometer. With those, you can determine how much the timing is advanced at various RPM's.
     
  4. Lonnie

    Lonnie 1/2 ton status

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    Crane sells an advance limiter that adjusts the total amount of vacuum advance. It bolts to the vac advance mounting screw. You can also put a small shim in the advance slot to limit its movement. A u-shaped piece of tin glued in place (in the advance slot) works well. I don't have a digital camera or I could post a pic of it.
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    Don't have any spare distributors around, but for a "one time, don't mess up" stock vacuum advance can, could you just grind a bit of metal away from the linkage that comes out of the can that is slotted?(if I remember the design right)

    That's of course not the right fix, just something I thought of.

    Really need to get in there with an adjustable mech advance setup...pinging at anything other than steady flat ground cruise should not be a vac. advance problem. For the time being back off the advance until it doesn't ping anymore when you drive.

    There have been some good timing related threads on here that would likely help in this situation.
     

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