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Best procedure for adjusting air/fuel mixture

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Dunc03, Feb 12, 2003.

  1. Dunc03

    Dunc03 1/2 ton status

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    Here is a question for all you carb tuners. What is the best way to set the air/fuel mixture on my Quadrajet. The thing is brand new( remanufactured by Holly) and it seems to be running a little rich. What is the best way to go about tuning this thing. I've already set the choke and fast idle, but I can't seem to get the fuel mixture right. Also has anyone had a carb that will idle itself up after its already warm? I figured it was just not clearing all the exhuast because the mixture was too rich when it was cold and then it burned better after its warm and idled up a little. Any ideas? The dang thing won't come back down after you rev it either. Is there a valve that is supposed to help with this, or is something just sticking. OK well that was more than one question, but I hope somebody will have the answers, cause I can't seem to figure it out. Oh yeah car info 86 Blazer originally had a 305 and has been up graded to a 330hp 350, the crate one from GM. It has a quadrajet four barrel and the only electronics on it is the one that has two prongs and is on the top passenger front. It has a wire that goes to the water neck. 700r4/208 I doubt this matters, but just in case. Thanks for the help in advance.
     
  2. schmuck

    schmuck 1/2 ton status

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    not sure if your cab is computer controlled, but normally you tighten the idle mixture screws one at a time until the engine starts to idle rough then back out about quater turn, after first time then reset idle speed and repeat procedure
     
  3. VitaminK5

    VitaminK5 Registered Member

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    I had an Edelbrock on the same crate motor. 1986 K5 and it would idle down to a near stall. New carb,manifold,headers,blah,blah. The electric heater for the choke with a grounding wire is what you are looking at I'd bet. Mixture with precision is with the exhaust temperature and mixture of gases. Pricey one time analyzer. Base idle ,no manifold leaks which could be the rise in rpm, and turning the fuel to stumble first with closer tuning by the air. PAIN IN THE ***. Get her to the shop and tune it on the machine.
     
  4. Dunc03

    Dunc03 1/2 ton status

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    The carb is not computer controlled, it only has the tempature controlled switch that I described earlier. It doesn't have any electronics that run into an ECM. The switch I was describing, I have heard calleed a Dual capacity switch. It works with cold starts and turns off after a certain tempature is reached. Thats what i have been told, but I haven't been able to varify it in any repair manuals, i.e Chiltons or haynes.
     
  5. Dunc03

    Dunc03 1/2 ton status

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    What kind off idle do you have with your setup. Mine idles real rough especially under load. Also what kind of exhaust system are you running. I thought the problem was back pressure from the stock cat, so I put on a high flow cat and I got more problems. THe thing used to would run in gear down to around 500 rpm's before it cut out. Now i can put in in gear around around 900 and it drops to about 600 when before it only dropped to around 700. Does this motor need more back pressure, or is something else to blame. It has a stock muffler, but that is pretty much hollow so it not real restrictive. Thanks for your help.
     
  6. rcpilot

    rcpilot 1/2 ton status

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    I hook up a vacuum gauage at the lowest point on the carb base or directly off a port on the manifold...then adjust one air/fuel mixture screw at a time until you get the best/most vacuum reading.. you can start by screwing the mixture screws in all the way and backing out 1 1/2 turns.. be sure to adjust one side then the other and back again...
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    http://yeagerd.home.mindspring.com/TablePics/M4MED.jpg

    Part number 260 is what you are talking about, and it IS the dual capacity pump.

    You might want to ensure that it is working correctly. I posted before about the way it operates (search for 170 or 170* on the board for the past month or two, and you should find it) I'd make sure that it's operating correctly. As I recall once the engine is warm, one of those wires should be getting 12V, but not when the engine is cold. Either that or the ground is what is switched, which would be easy to test as well.

    Idle mix screws: in is richer, out is leaner. After a rebuild, I start with them all the way in. Then back them out either 3 or 4 turns. That has ALWAYS worked to get the engine to at least idle. From there either in or out, whatever it takes to get the idle smooth.

    You have to do one side at a time, and have some patience. In or out on one side until the engine starts to stumble, turn the screw the opposite way until the engine stumble JUST goes away, then back to the other side, and repeat.

    Vacuum way as I recall is how GM says to do it, if you have the equipment to do it that way, try it. I never have, but doesn't mean I'm right either.
     
  8. Dunc03

    Dunc03 1/2 ton status

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    Thanks for that link. I've never seen that exploded view of a quadrajet. I have both a Chilton's and Haynes' and neither has a picture that has the dual capacity pump on them. Maybe I just over looked it. Where did you pull that picture from. I saw your previous post about the pump, I'll go back and review it in more detail. I think I might try the vacuum way of adjustment since I have a vacuum gauge. If that doesn't work I'll try it the old fashioned way. That probably works better anyways, I'm just very inexperienced when it comes to tuning carbs. Thanks again for the help.
     
  9. m j

    m j 1/2 ton status

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    when ever someone posts a carb question the responses are usually to play with the idle mixture.
    this has little if any effect on jetting.
    do most not know about jets?
     
  10. dyeager535

    dyeager535 1 ton status Premium Member

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    I guess the question should be asked: is it rich at idle or is he talking about while cruising?

    Based on the idle problem, I assume he is talking about idle being rich, which is the mix screws of course.

    I wouldn't trust the holley having the right setup for his specific engine though, at any engine speed. I'm sure there is plenty of improvement to be had by fine tuning the primary jets/rods and power piston spring...that kind of stuff can be covered by Doug Roe's book.
     
  11. Dunc03

    Dunc03 1/2 ton status

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    Actually I don't know alot about jets, but I don't think that it is the problem. The reason why is this engine is test with a 650 cfm carb and mine is a 795, so I don't think it is lack of fuel flow. In this one case adjusting the air/fuel mixture did help. The carb was new and had not been adjust to meet the needs of this engine. the carb was set for a stock engine and mines about 150hp from stock. Jets still might be a problem i don't know cause its still not running as well as I think it should, but it did smooth out alot at idle. Thanks for the help guys.
     

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