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Crazy Vacuum advance

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Blue85, Jul 9, 2003.

  1. Blue85

    Blue85 Troll Premium Member

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    This is weird. My idle has been getting progressively worse over about the last month. This week, I noticed some knocking when I would let up on the gas for the 1-2 shift ("hey, who has the diesel?"). My guess is that when I closed the throttle the vacuum went up enough to stack some vac advance on top of the mechanical advance and put it into knock land.

    (I was planning on figuring it out this weekend and driving the car, but the fuel pump in that quit yesterday and so...) When I hit it with the timing light, I was at 45* BTDC at idle. /forums/images/graemlins/confused.gif Unplugging the vacuum dropped it back to about 15*. My vac can was giving me 30 degrees! I have no idea how this came to be. When I set it all up a year or so ago, I had 14* initial, 20 degrees mechanical and I thought about 15 from the can. This is the Crane adjustable kit.

    WHat confused me is the vacuum limit cam. This thing doesn't limit the maximum vac advance, but the minimum vac advance. It holds the thing activated slightly. However, this can still be used to limit the amount of vac advance by setting the cam and then retarding the initial advance back to where it was to begin this. In fact, this is necessary since setting the cam is like always having a little bit of vacuum advance present. The end effect is the same, because increasing the minimum will reduce the total range of vac advance available, but this takes a lot more steps to get everything set right.

    Has anybody else run into this? Is there another way to use the limit plate?

    The idle is smooth now and no knock. /forums/images/graemlins/grin.gif I just wish I knew why things have changed.
     
  2. Blue85

    Blue85 Troll Premium Member

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    So nobody has ever used a vacuum advance limit cam before?
     
  3. Mastiff

    Mastiff 1/2 ton status Premium Member

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    [ QUOTE ]
    So nobody has ever used a vacuum advance limit cam before?

    [/ QUOTE ]

    Never heard of it! /forums/images/graemlins/tongue.gif

    Maybe you should just replace the vacuum advance kit? I have one of the Accel kits, but it hasn't busted yet or anything.
     
  4. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    Those Crane units are wierd little pieces I tell you. First off, it looks like one company makes those adjustable vacuum advance cans for everyone. They've all got AS-20 stamped right on them. I actually found a brand new one in a bucket full of junk. It still had the little rubber nipple on it. What I found interesting with mine is that inserting an allen wrench and turning it to the left dropped off about 2 degrees per complete turn. I put on a vacuum gauge and didn't see any big effect on rate. /forums/images/graemlins/thinking.gif

    Maybe the Accel units are made different, because they don't come with that plate. I tend to stay away from those aftermarket weight kits too. Same story, same part sold under different names. Stock GM weights and springs from a LATER model HEI will work great. I know *gasp*, all the advertisements and "experts" are actually wrong about those stock weights being to heavy. The later model weights don't use bushings but have this wierd little built in insert around the pivot point. They're very slow to wear. With those weights and the heavy stock springs the mechanical advance begins just off idle and is in all the way by about 2,800 rpm. Thats about the best generic mechanical advance setup you could want!

    If I hadn't found that vacuum advance can in that junk pile. I probably would have hit google and got an assortment of GM part numbers for 10, 12 and 14 or 15 degree cans. That, or make my own limiter. I love my adjustable can though. I've gone from a manual with 3.08's to a 700r4, then a 700r4 with 3.73s with stock stall, and now to a 2,000 RPM stall, it sure makes life easier being able to adjust fast.

    I don't know much to tell you since you fixed your problem, but 14-16 initial, with 20 mechanical in by 2,500-3,000 and about 10-16 degrees vacuum advance to give best part throttle response and highest cruise vacuum. Just have to play with that last one. Oh, and use ported vacuum, its was designed for just such a purpose.
     
  5. Blue85

    Blue85 Troll Premium Member

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    I am using the stock weights with lighter springs. When I made this change, the increase in torque was noticeable. I guess I will keep running this can. I have not been able to meausure any changes of rate from adjusting the screw in the CAN, but it seems that the total amount of advance is not changed. I was only able to change that by using the little cam. This is inline with the information from Crane.

    Have you ever wondered why someone tossed your advance can in the junk? /forums/images/graemlins/grin.gif
     
  6. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    Never question the junk pile! I found a brand new 73-87 heater core still in the box over in a pile of junk. It now resides in my '83 will roast your butt off and not leak a drop. Also found a 3.73 12-bolt in an iron pile one day. All I had to do was extract and broken carrier pin bolt and away I went.

    I'm bettin' Accel and Mr. Gasket vac advance are probably a little different than the Crane one. Regardless, the only way messing with the advance is going to be efficent is you don't have the whole smog setup hooked up.
     
  7. Blue85

    Blue85 Troll Premium Member

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    I'm pretty sure the adjuster screw on the advance can is just preload on a spring. The spring is pushing the plunger one way and the vacuum diaphram is pulling it the other direction. I would expect that as you crank up the preload, it would take a lot more vacuum before it starts to do anything. If it really adjusts the rate (mm movement per inch Hg or whatever) it is probably cutting off either the top or the bottom of the range while doing it. I have found that the total amount of movement is unaffected by the screw, so it must just be sitting still until more vacuum is present before moving.
     

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