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Q-Jet Adjustment (Power Piston)

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Blazer_Boy, Jun 11, 2002.

  1. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    Got a question on the M4ME Quadrajet for 1981-86, or Edelbrock Q-Jets. These carbs have a little dome shaped bulge on the front passenger side. Inside of the carb next to the power piston is a little stop. If the carb were on the vehicle, the stop would be closer to you than the power piston. What do you set this thing to? Is this the Adjustable Part Throttle or something else? From what I can see, the job of it is to keep the metering rods slighty out of the jets to deliver a little more fuel. How many turns out should this be? I never see any mention of this on diagrams of Q-Jets, yet I've seen this on three of them, incluiding a Q-Jet I robbed of an old 403.
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    I can't picture what you are describing..do YOU have pics? It almost sounds like you are describing the altitude adjusting carbs, (just because you mention the bulge) but that setup, once the air horn is off, is pretty apparent, because underneath that "bulge" there will be an assembly, as opposed to just a black plastic spacer on a "standard" carb.

    I've only seen one or two of these (I had or have one somewhere) but they were made for high altitude use. Is this not what you are describing?

    The only other "stop" I can think of is the little pin that sticks out of the power piston, which I would imagine would be something like a lean stop. (since if it is preventing the rods from going down, it would limit how lean the primaries could go)
     
  3. Jay73K20

    Jay73K20 Registered Member

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    I believe three full turns for a starting point. You can also call Jet performance tech line and tell them you had your carb rebuilt by them (lie) and you were messing with the primary rods adjustment screw and screwed it up. Ask them where you should start. I thought it was to adjust power mode not cruise mode which would limit the metering rods from coming fully out of the jet. The stock ones have a funny looking head and the only way to adjust them is to pull the air horn off. Jet performance drills and taps the air horn and puts a phillips head machine screw in place of the special stock screw with a plug in the air horn so it is easily accessible. I would speculate that adjustment would only affect power mode A/F mixture and power mode to wot transition.
     
  4. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    You got it! I don't have anything in the aneroid cavity (area under bulge). Some have a plastic deal, some don't, and my edelbrock kit has a diagram with something in there that looks like a beehive, lol.

    I've heard that the stop that limits that pin off the power pistons can adjust part throttle cruise, I could be wrong though. I like the idea below about calling Jet. I just wrote a letter to Edelbrock and did a little brown nosing because this carb is calibrated exactly the same as their #1904. I also have a bunch of Edelbrock stuff, so I'm not really taking advantage of them. I had an idea the other day and just bought some supplies. I can pop a plug out of the top of the airhorn and adjust it without removing everything else. In its place I bought a 3/8" flat allen head plug. It looks almost factory.
     

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