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Quadrajet swap

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by TrcksR4ME, Jun 25, 2003.

  1. TrcksR4ME

    TrcksR4ME 1/2 ton status

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    How stupid is this...swapping a GM part that the truck came with back onto it... /forums/images/graemlins/grin.gif

    I am gonna be putting a q-jet on my truck to replace the Carter POS that is on there now. The manifold I have looks to be a square bore (edelbrock performer rpm). Will an adapter work ok to fit the q-jet on? I really do not want to mess with manifold replacement at this point.

    Also the q-jet has like 6+ vacuum ports on it, and currently I only have 3 vacuum lines hooked up on the Carter POS. Is there a good diagram somewhere (or does someone have one?) that shows what the different vacuum ports on the q-jet are for?
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    Kind of imprecise, but here goes /forums/images/graemlins/smile.gif

    Look inside the carb primaries. See the slots in the walls? (there will be one hole in each near the base, thats idle mix, ignore those) Plug one of the slots with your finger, (if you can reach them) and apply pressure (blow into)each of the fittings on that side of the carb, one at a time obviously. That eliminates all the "ported" or "timed" vacuum slots. If you've got two of those, one's going to be EGR and ones vac advance more than likely.

    Everything else, not setup for ported, is manifold, except any that are in the top 3rd of the carb, and usually on the back. Those (if present) are usually used as a source for filtered air.

    Think that covers all of them.

    Usual disclaimers apply, Q-jets were "only" used for 25 years, so there are literally hundreds of variations on the basic design.
     
  3. TrcksR4ME

    TrcksR4ME 1/2 ton status

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    think an adapter will mount the carb ok, I looked at one that has the four holes versus the one large hole and the only problem I see with the one with the four individual holes is that the larger secondary holes bottleneck to a smaller size...if that makes sense...and I would think this limits airflow. It looks like there is enough metal there to customize the adapeter for better flow, but is it worth the time?

    I heard using the adapter with one large hole is not as precise since each "barrel" of the carb doesn't feed only to a specific area (for lack of a better term). /forums/images/graemlins/confused.gif But may be the better option at this point.
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

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    I dunno. Usually adapters mean half-rearend, but of course, if you've got one around or just can't swap intakes.

    Same deal with sharing the "plenum area"...some motors like a small slot between each side, some don't. Some gaskets seperate all four "barrels", some don't.

    GM played around with this a fair amount too, so it's not something you can't try. From what I recall with the square bore carbs, at least most of the smaller ones have secondaries just as small as the primaries, and if the intake is set up for that design, obviously there will be problems mating that up with the Q-jets huge secondaries. Most adapters I've seen use "beveled" ramps to direct the air/fuel mix, but any additional bends and metal contact for the air/fuel mix is going to typically hurt power, but how much, who knows? That's the tradeoff though, adapter vs. intake.

    I'd at least try the carb first and see if you are going to like running it, and if so, then look into getting the right intake. It's not like Q-jet intakes are hard to find or anything. /forums/images/graemlins/smile.gif Maybe someone on here (parts wanted? : )has an adapter they can sell you cheap or even loan you. It's not like they would wear out, and there must be millions of them floating around not being used.
     
  5. TrcksR4ME

    TrcksR4ME 1/2 ton status

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    The right intake would be the best way to do it, but Im kinda cheap /forums/images/graemlins/grin.gif

    I'll try a cheapie shmucks adapter for $20 and if it sucks(which it should /forums/images/graemlins/smile.gif ) err doesn't work well ill go from there. Can't be worse than a Carter POS...I hope /forums/images/graemlins/crazy.gif
     
  6. blasterD

    blasterD 1/2 ton status

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    If you have the regular performer RPM Part #7104 It should be casted and machined for both spread bore and square bore carburetors. I think all Edelbrock Performer/Perfomer RPM are machined this way.
     
  7. COCHEV

    COCHEV 1/2 ton status

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    another prob you might encounter is the choke. what kind do you have? if its the devorced once you intall the adaptor and then the carb, the choke leakage wont work. i'm not sure that your manie even has provisons for a devorced choke. i have this prob now so i just baught a q jet with an electric choke. or you could rig up a manual choke. i've done this as well. saves having to buy a new manie. i'd keep the RPM. and you'll love the q jet. good luck.
     
  8. TrcksR4ME

    TrcksR4ME 1/2 ton status

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    One last question(s) for you...

    On the choke side of the carb,

    A) there is a vacuum valve thing at the rear of the carb, what does this hook to, manifold vacuum?

    B) hooking up the electric choke, I think the original plug for my truck might still be there somewhere, but if not is this hooked up to a ignition power supply? And does the electric choke always draw power then, probably not much but just wondering?
     
  9. TrcksR4ME

    TrcksR4ME 1/2 ton status

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    [ QUOTE ]
    If you have the regular performer RPM Part #7104 It should be casted and machined for both spread bore and square bore carburetors. I think all Edelbrock Performer/Perfomer RPM are machined this way.


    [/ QUOTE ]

    You can get it either way, and why would one get the square bore only one? I dunno, the dumb [naughty word] who I bought the truck from did a lot of weird things...
     
  10. dyeager535

    dyeager535 1 ton status Premium Member

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    [ QUOTE ]
    there is a vacuum valve thing at the rear of the carb, what does this hook to, manifold vacuum?

    [/ QUOTE ]

    What does it do? I'm assuming this is a choke pulloff? (linkage hooks in under the choke coil, and perhaps to the secondary air valve?) If so, yes manifold vacuum is what it hooks to. Again this is one of those "they had 5 different setups" so I'm not sure which you are dealing with.


    [ QUOTE ]
    hooking up the electric choke, I think the original plug for my truck might still be there somewhere, but if not is this hooked up to a ignition power supply? And does the electric choke always draw power then, probably not much but just wondering?

    [/ QUOTE ]

    Electric choke has an oil pressure switch inline so that it isn't getting voltage when there is no oil pressure. (only works when the truck is running basically) Any chev q-jet equipped truck from '81-86 (or so) will have that switch on it, as well as the "t" fitting for the sending unit.
     
  11. TrcksR4ME

    TrcksR4ME 1/2 ton status

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    [ QUOTE ]

    Electric choke has an oil pressure switch inline so that it isn't getting voltage when there is no oil pressure. (only works when the truck is running basically) Any chev q-jet equipped truck from '81-86 (or so) will have that switch on it, as well as the "t" fitting for the sending unit.

    [/ QUOTE ]

    /forums/images/graemlins/1zhelp.gif

    That sounds complicated, what would a late 70's Blazer have had for a choke set-up and how would I make an electric choke work? I almost want to rig up the manual choke LOL...

    I am starting to see why people yanked these things off in the first place /forums/images/graemlins/tongue.gif
     
  12. COCHEV

    COCHEV 1/2 ton status

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    all you need is 12 volts when ignitin is on only for an electric choke. mid 70s righs mostly had the devorced choke that worked off manifold heat. i don't think you can convert to electric. your best bet is another carb with elecric choke or convert to manual. either way, its worth it to have a q jet. they are the next best thing to fuel injection. /forums/images/graemlins/thumb.gif

    by the way, e bay is a good place to pick up cheep used q jets. as is the junk yard.
     
  13. dyeager535

    dyeager535 1 ton status Premium Member

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    I doubt its very complicated, there is probably a 12V constant source in, and a 12V switched source out. Total of two wires, but really, just a wire with a break in it. Not complex at all.

    Even lawnmowers don't use manual chokes anymore /forums/images/graemlins/tongue.gif
     
  14. speed_deed

    speed_deed 1/2 ton status

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    I agree with you to swap off the carter but... Edelbrock 1406's are the best thing I have used on my k series. I have used carters and q-jets but none really cumoare to the Edelbrock. I'd chevk into them first. I recently just yanked all of my emissions junk off of my '85 and put a piece of metal off of a street sign over the EGR, I dont know if you can fo this where you live but, I 100% DONT regret it!!!! I got a couple stock q-jet manifolds laying around if you want to trade. Just put some little rubber caps over your vaccum holes on the carb and plug them on the other end.



    BTW I have two whole q-jets for sale and 1 in parts .
    I also have a carter if you want to give it a second shot.
     

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