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Secondaries, when do they open?

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

  1. sled_dog

    sled_dog 1 ton status

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    I took my air cleaner off and cleaned up my carb and all. I kept hitting the throttle and noticed my secondaries never opened. I held it at least 3000 RPMs for a little and they still didn't. When do these suckers open?
     
  2. sdavid

    sdavid 1/2 ton status

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    What carb you got? If it is a Quadrajet, the secondaries are vacuum operated. So maybe you have a vacuum problem. I would think about half throttle, but it does not take as much throttle to get 3 grand in park as it does in drive.
     
  3. sled_dog

    sled_dog 1 ton status

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    according to my dad thats just the way the rochester carb is. Says it opens gradually. I don't know much about this so I am gonna just believe it works under load /forums/images/graemlins/smile.gif.
     
  4. R72K5

    R72K5 Banned

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    they wont flip until there is enough vacuum under the throttle valve flaps to make them flip open, there is adjustment that can be made on pass side of carb if you loosen up the lockscrew up under the lip of the air horn casting there and turn the flat blade screw in the end of the secondaries valve shaft, they are operated literally by vacuum- completely, actuated by the vacuum of the air wanting to get into the intake at higher RPM range
     
  5. wrathORC

    wrathORC 1/2 ton status

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    They're vacuum operated. They aren't going to want to open at a steady RPM and certainly not when the motor has no load on it. Push down on them to see if they open. There might be something stopping them from opening (bent linkage or something). There's a vacuum can that is used to slow them from opening (up to 3 seconds).

    Also, you'll need to hold it open at WOT to get them to open if you've got your head under the hood instead of driving it. The best way to get them to open is to do several short stabs (a second or two long).

    The blades for the secondaries don't start opening until you reach about 35° on the primaries.

    Some people modify the secondaries so they fling open easier and then use the vacuum can to coordinate when they open (3/4 of a second) so as to avoid "quadrabog". I like to keep the spring at the stock stiffness and eliminate the vacuum can. What happens with the soft spring tension is the blade will fling open, slam shut, and bounce back open creating "quadrabog".
     
  6. sdavid

    sdavid 1/2 ton status

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    There are some expert Rochester (quadrajet) guys on here that can tell you for sure. /forums/images/graemlins/thumb.gif

    Looks like they already did.
     
  7. imiceman44

    imiceman44 1 ton status

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    /forums/images/graemlins/smile.gif
    I have been there, and I thought there was something wrong with my carb, but since I had a van, I took off the cover and drove it while looking at the carb.
    It will open under load when needed, and surprizingly not very often.
    I did change the preload on the spring and I got it to open more often and I got more power but my mileage dropped drasticaly /forums/images/graemlins/frown.gif
    I have a 79 and I had the rebuild kit covering 73-80 and for 75 th epreload was 3/4 of a turn, and for 79 it 7/8 of a turn so i went with the 75 setting.
    I like the new found power.
    /forums/images/graemlins/thumb.gif
     
  8. dyeager535

    dyeager535 1 ton status Premium Member

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    Secondaries open based on airflow, not vacuum. They are held *shut* with vacuum. Try pushing the secondary air valve (top rear butterflies, the big ones) open with your finger while the engine is running. Unless your engine makes no vacuum, it will be very hard to push open even a hair.

    Once engine vacuum drops, the vacuum brake lets go of the secondary air valve shaft, and allows it to open based on engine airflow demand.

    You can get secondary vacuum breaks with larger holes to decrease the amount of time it takes to let go of the air valve, or you can drill it out, but it is precise. Too much and you've ruined it. Messing with the secondary air valve spring is a good way to address it instead, that spring sets tension (try opening it with the engine off, thats why they shut when you let go) and that can be adjusted. I set mine as loose as I can, while making sure that the air valve will close EVERY time I open it with my finger, and release it lightly. Too loose though, and you can have the air valve bouncing around, which isn't good.

    Doug Roes book "Rochester Carburetors" (HPBooks) is a good one to read, pretty interesting.
     
  9. TorkDSR

    TorkDSR 1/2 ton status

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    not to disprove what the last 6 peeps said, but mine, and my spotter's quadrahogs are mechanical... they're both on mid seventies engines. his on a 454, mine on a 400. you can move the throttle linkage and watch the secondaries open. course you have an 84...if its original it should be opperated by engine airflow demand.
    Ryan
     
  10. JIMs70K25

    JIMs70K25 1/2 ton status

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    My q-jet secondaries are mechanical also. The secondary throttle plate is connected to the primaries. As far as the air valve opening I believe it will only open when the throttle plates start to open. Definetly read Doug Roe's book. It talks about how to modify the linkeage between the primaries and secondaries to get the secondaries to open sooner. Involves changing the tension in a spring on the secondary end of the linkeage. You can not bend the linkeage to get the same results, you will actually cause your secondaries to be open during idle(been there, done that then read the book).
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    I think the whole thing of calling them "mechanical" vs "vacuum" is a bit of a misnomer, but comparing an Edelrcok "mechanical" carb to a Q-jet, *all* Q-jets are vacuum.

    The only difference being that on the Q-jet, if there is not enough demand for increased air flow, coupled with the high vacuum, the secondary air valve won't open. No extra fuel, no extra air. With "mechanical" secondaries, you would force the air valve open, and Q-jets aren't setup that way. It's very hard to "overcarb" with a Q-jet for this reason, which is why there was only 750 and 800CFM models made, some of which went on I and V6 motors, some of which went on 454CID+ motors.

    You would be correct in saying the secondary throttle plates are mechanical, but the secondary air valve is most certainly not.
     
  12. Blazer1970

    Blazer1970 1/2 ton status

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    To try to make this a little more clear, Q-Jets have two sets of butterflys on the secondaries. The one you see on the top is the air valve, and it will only open when two conditions are met:

    1. The secondary butterflies underneath it have to be open. They open based on the position of your foot on the pedal, but they are locked closed until the choke is fully open (unless your carb is modified).

    2. Manifold vacuum has to be below a set level.

    Since manifold vacuum is always high when you are revving the engine under no load in the driveway, the air flapper valve will NEVER open under these conditions. If you try to adjust the carb so that the flapper opens in the driveway, you will have it screwed up big time. If you think about it, this makes sense since you never need a lot of fuel to rev the engine under no load.

    As noted above, the actual secondary butterflies are mechanical, and they open when you put your foot to the floor. In the driveway, you have to push open the air flapper valve with your finger and look down inside the carb in order to even see the actual secondary butterflies.

    If a Q-Jet is working right, when you put your foot to the floor while driving, there will be a slight delay as the engine builds a few RPMs and the vacuum drops (since the engine is under load and not free revving), and then the air flapper valve will pop open. At this point you will feel a surge of power, and the exhaust tone will get a lot louder, and you will be having fun.
     
  13. wrathORC

    wrathORC 1/2 ton status

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    How can the secondaries not be operated by vacuum? What on earth sucks the top air valve open? Magic? Sure, some goofball can say they're mechanical because the secondary throttle valve is connected to the primaries but all the metering work is done by the air valve.

    The reason why you can't get the air valve open is because of the vacuum can. A proper vacuum can setup actually hooks both to the bottom butterflies (throttle valve) and to the air valve. I ain't got no stinkin' linkages hooked to the secondary air valve or throttle valve so mine will open right now with the truck parked and cold.

    For the price of a set of orifice drillbits for a torch you can buy an adjustable vacuum can and not have to worry about drilling too big of a hole.
     
  14. Blazer1970

    Blazer1970 1/2 ton status

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    Well, the air valve is more or less operated by vacuum since that is pretty much the same thing as air flow. The operation of a Q-Jet is different from what is normally referred to as a vacuum secondary four barrel though since that type of carb (Edelbrock, Holley etc.) use vacuum to control when the secondary butterflies open, and they do not have a separate air flapper valve.
     
  15. JakeInAZ

    JakeInAZ Registered Member

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    When do they open? If you drive like me they're open at all times /forums/images/graemlins/wink.gif In all seriousness though, it is a vaccuum problem of some sort assuming that you're able to easily open them manualy with your finger. If it's hard to open them with your finger then it's a whole other sticky situation and you're lucky they don't open.....cause if they're just sticking then be glad they're not sticking open /forums/images/graemlins/wink.gif
     
  16. twenty_below0

    twenty_below0 1/2 ton status

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    I may be wrong but I agree with you I had a q-jet off of a 455 pontiac and it was also mechanical just like an Edelbrock or Double pumper! period ! NO CHOKE pull off all manual linkage, you could bust it off in the drive way with out load any time you wanted and I wish I could find another like that the vacum style SUCKS...but that's my tiny little opinion and what do I know right I'm still considered new to the sight /forums/images/graemlins/rolleyes.gif! It could have been modified I suppose but the guy I got it from (and paid alot for it) said there weren't alot of them around, I tend to listen to the old grey haired, John Force type gear heads because experience is worth so much when it comes to mechanics /forums/images/graemlins/grin.gif
     
  17. wrathORC

    wrathORC 1/2 ton status

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    So you're saying that the secondary air valve is directly connected to the throttle?
     
  18. twenty_below0

    twenty_below0 1/2 ton status

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    I cant remember how it was set up but yes it was full manual....I wish I had access to the carb so I could down load a pic. of it. I do know like I said there was not a choke pull off like most of the Q-jets I've seen on Chevy's. Maybe someone else has had an experience with a Pontiac version and I suppose it could have been converted but from what I was told it was stock ??? /forums/images/graemlins/confused.gif
     
  19. wrathORC

    wrathORC 1/2 ton status

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    There's all sorts of automatic chokes. There's ones with a coil and a linkage that rests on the intake (which is what I'm running now). There's ones with heat riser tubes from the intake that go to a coil on the side of the carb. There's electric ones. There were other versions but I've never seen them on a Quadrajet, just monojet and 2 barrels.

    There's a manual choke too. The kind where you sit in the vehicle and operate it via cable.

    I have never seen a Quadrajet (made by Rochester, Carter, or Edelbrock) that had the secondary air valve hooked to the throttle. That means moving the throttle opens the top air valve even when the vehicle is off.

    I've seen quite a few Poncho carburetors (ok, maybe only ten) and I've never seen one like you describe. Sure, I've seen manual chokes. My Dad used to work on cars for a living in the 70s and he was a Poncho enthusiast but he says he's never seen one.
     
  20. dyeager535

    dyeager535 1 ton status Premium Member

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    </font><blockquote><font class="small">In reply to:</font><hr />
    How can the secondaries not be operated by vacuum? What on earth sucks the top air valve open? Magic?

    [/ QUOTE ]

    </font><blockquote><font class="small">In reply to:</font><hr />
    The reason why you can't get the air valve open is because of the vacuum can. A proper vacuum can setup actually hooks both to the bottom butterflies (throttle valve) and to the air valve.

    [/ QUOTE ]

    Soo...vacuum opens the secondaries AND keeps them closed? Please explain!
     

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