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Can we settle this once and for all... Vacuum advance line....where??

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Ben, Jan 28, 2004.

  1. Ben

    Ben Registered Member

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    I have read many a post about where the vacuum advance line gets connected to and almost all the responses offer different opinions it seems. Concerning my 84 K5; 84 Q-jet on a 350. When I hooked up a vacuum gauge to the ports, only one port has vacuum. Three small ports on the front of this carb, two on either side of the carb at the base and one toward the top of the carb. The two ports at the base of the carb have no vacuum at idle. The upper port has vacuum at idle which is where I connected the vacuum advance line to. Someone mentioned to me "don't connect it their, that's full time idle." Another person tells me, you want full time idle. What gives? Running problems? Well, sort of. Manual transmission. Granny gear and 1st, 2nd and 3rd. If I drive in 3rd gear around 40 MPH, the engine stumbles. Speed up to over 55 and into 3rd gear she goes and runs fine. The same with 2nd gear, below 30 MPH or so and it stumbles. Never did this before until last week. What did I do or change? Nothing! Just the gremlins taking over it seems! So, I'm thinking that full time vacuum might be causing the problem.... but the other tow ports don't have vacuum at idle. So, any ideas? Let it remain connected to the vacuum at idle port? Timing is 8 degrees.
     
  2. big83chevy4x4

    big83chevy4x4 3/4 ton status

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    all of mine i have put on ported vacuum and that is all that i have seen. i dont know why anyone would tell you that you want full time vacuum, its totaly opasite of what you are trying to do. all mine are hoocked and the small port on the driver side of the carb near the base.
     
  3. Ben

    Ben Registered Member

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    As I posted; some of the replies concerning the same question posted in the past have stated just the opposite of yours; vacuum at idle is what is needed. I'm connected to vacuum at idle. The other ports have no vacuum at idle. Once I give the throttle a turn, then I have vacuum, but again, not at idle. So, I'm still confused! If the port has vacuum at idle, then it's full time/constant vacuum, right?
     
  4. bigblock454

    bigblock454 Clack Clack Clack Premium Member

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    Yes, you want it on a full vacuum source. This is the same functionality as all computer controlled vehicles and why you have to disconnect a wire when timing the engine. Disconnecting the wire is the same as removing the vacuum line.
     
  5. NerdBoy

    NerdBoy 1/2 ton status

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    Odd. According to my manual for my Edelbrock carb, you are supposed to hook it up to ported vacuum.
    Full-time vacuum will force you to mechanically retard your timing to keep the idle speed correct. With your timing retarded, that can cause problems with starting. Notice that I said it "CAN" cause problems with starting. It won't on all vehicles, depending on what size carb, what type of cam, and a few other factors. When I had my old .272 cam in my engine, it didn't matter where I hooked up the vacuum advance, as there was so little vaccum at idle that it didn't pull the advance in at all. When I pulled the cam and replaced it with a much milder cam, I had to switch to ported vacuum, as the vacuum was 12psi at idle.
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    305 and 350 emissions decals

    You can see that both are hooked to the same port. Of course, if everything has been removed that controls the feed to vacuum advance...
     
  7. blasterD

    blasterD 1/2 ton status

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    I would agree that logically you would think that ported vaccuum would work better, but I have found that the truck gets better mileage and has better throttle response with full time vaccuum. Maybe you should try it both ways to see which you like best. /forums/images/graemlins/waytogo.gif
     
  8. Mastiff

    Mastiff 1/2 ton status Premium Member

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    I'll be happy if this is ever gets fully resolved! For now I have mine on full time manifold vacuum. It makes sense to me because you can tolerate more advance when the engine is not under load. When you gun it, it retards some to avoid detonation.

    What would ported vacuum do? You'd still have very little under heavy load, and none at idle. You'd get it at open throttle when there wasn't much load? Do you really want even more at that point when you already advanced it for idle?

    I can see the argument that you'd want the advance while starting and it wouldn't be there yet.

    Good thing I'm switching to TBI soon and it'll just go away anyway!
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    It really doesn't matter, *except* for idle.

    Vacuum advance is an economy device for constant throttle cruising. There is none when vacuum drops or is non-existent, which is starting the vehicle and acceleration.

    If your engine can tolerate advance at idle, with no ill effects, run it. If it can't, run ported. It really has no bearing on anything else, because it isn't really doing anything except during cruise.

    If you really think that it makes a difference, hook up a vacuum gauge to the engine and take it for a spin. You'll see exactly what I saw. Cruise and idle are the ONLY times you have high vacuum readings. Even climbing a small hill drops engine vacuum (and thus vacuum advance) to nothing.
     
  10. SUBFAN

    SUBFAN 1/2 ton status

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    My vote is for full time vacuum....
     
  11. NerdBoy

    NerdBoy 1/2 ton status

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    [ QUOTE ]
    If your engine can tolerate advance at idle, with no ill effects, run it. If it can't, run ported.

    [/ QUOTE ]

    I think that is the best advice so far.
     
  12. 88Silverado

    88Silverado 1/2 ton status

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    UUgh....Ported normally takes vacuum above the throttle plates and full-time below the plates. On ported, when idiling, there is very little air flowing in the upper part of the carb so no vacuum. When the plates open, more airflow down the top creates more vacuum on the port. ported will jump timing up very quickly on initial depression of the accellerator but drops off equally as fast as full-time does during accelleration. The mechanical advance pulls in the timing during accelleration. This helps emissions by minimizing timing advance at idle and cruzing but is less efficient. It also can add to pinging when under slight load because the slightly cracked throttle plates will apply some vacuum pulling in more timing when you dont need it. As you accellerate harder, the pressure thru the carb equilizes and drops removing this advance.

    Full-time pulls air from below the throttle plates and will have a much higher vacuum when the throttle plates are closed or slightly cracked while cruzing. This pulls timing in at idle and while crusing. The engine will run smoother at idle on full-time because the timing will be around 35-45 degrees, but emissions will be worse (stinks). At cruze speed, the engine is under little load and pulling in max timing making it run more efficiently. It also reduces pinging because any increase in the opening of the throttle plates will cause the vacuum to drop removing timing reducing the chance of pinging. Up until about 72ish everything was on full-time, then came the smog laws and they switched to ported.

    During normal accelleration, neither one is providing any advance because both ported and full-time are seeing the same low pressure above and below the throttle plates allowing mechanical (or ECM I guess) take over.


    If you have smog requirements, run ported. If not, run full-time for better gas mileage.
    Heres a pretty good graph
    [​IMG]
     
  13. Blue85

    Blue85 Troll Premium Member

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    This is always a big discussion. In the end, nobody here will be able to tell you the best way to set up your truck. You'll have to experiment and find what's best.

    Yes, the vehicles all came with full vacuum to the distributor. If everything else is mostly stock, this will probably still be fine. The problem with the stock setup is that it's more for economy than power and should let you run really low octane without pinging. In other words, it's boring. Generally, you need quicker mechanical advance and less vacuum advance than the factory setup for good performance and economy.

    Like Dyeager said, the only difference is at idle. I have a vacuum gauge in my cab and I have driven around with it connected to ported and full vacuum. Once you touch the throttle, they are the same. In my case, I run the initial fairly fast (10-11 degrees) and I have a Crane adjustable advance can. In this setup, I was getting too much advance at idle and it was actually making my idle a little rough. I moved the vacuum advance to ported and now it works great.

    Bottom line: don't do it because somebody tells you to. Go try both ways for yourself. It takes about $1.00 worth of vacuum hose and fittings and 5 minutes to change it.
     
  14. Goober

    Goober 1/2 ton status

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    /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif

    Cool graph. That helps explain it a little better.


    /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif
     

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