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  • highway woes

    I have a 77 cj5 with a 304ci and an edlebrock 600cfm carb with off-road needles and springs and have noticed at highway speeds the jeep lacks power and vacuum is low and it also misses or seems like it isnt getting fuel. The jeep runs fine at low speed other than a occasional backfire. I dont know a whole lot about carbs and vacuum systems so any help or advice is appreciated.

  • #2
    Where do you have the vaccume off the distributor hooked up to????

    High5

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    • #3
      my experiance with carbs has been when the engine runs that way, it is a vacuum leak.
      in my 80 I was able to cap all the vacuum ports and eliminate the vacuum system completely. but I also have a lot of um, rigging, done.
      it fixed the problem.

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      • #4
        I was thinking that he had it hooked up to manifold vaccume and not to the carb base. That wouldn't allow the distributor to advance like it should.

        My 0.02

        High5

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        • #5
          on the edlebrock carb it is hooked up to the passenger side vaccuum port and the drviers side is capped off but it is hooked from the carb base straight to the advance on the motorcraft distributor.

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          • #6
            What is the gauge say? I cannot remembewr what it is supposed to be. Maybe someone with a FSM could give you the numbers.

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            • #7
              The vaccuum is supposed to be 12-15 and it is but not at highway speeds and not under load. I am begining to think maybe I should just buy a new distributor and be done with it.

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              • #8
                You won't get 12-15 inches of vacuum under load or at full throttle, just the way carbs work. At idle you should easily get between 15 and 18 inches depending on your elevation and the condition of the engine. As you open the throttle you expect the vacuum to drop off steadily until it reaches zero at WOT (Wide Open Throttle).

                Double check your vacuum diagrams and see if the correct port on the stock carb is above or below the throttle plates and then make sure your new carb is using a port located at the same spot (under or over the throttel blades).

                Does it really backfire or is it just an occasional hiccup or miss? A real backfire (loud bang) would indicate a possible vacuum leak or improper timing. An occasional miss might indicate a lean mixture (hard for me to tell without hearing it run and smelling the exhaust, both of which are difficult to do online).

                A 600 CFM is about right for a 304, but you may have to rejet the carb to get the most out of the motor.

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                • #9
                  If you have the edelbrock 1406, which is the 600cfm electric choke. This carb is jetted for a lean fuel economy fuel mixture that my 304 hated! Edelbrock at the Jamberee gave me a couple new rods and springs to try and my Jeep runs so much better and mor powerful.

                  I used to have to advance timing to try to create more power, now I have my timing set at 10BTDC and the motor is quieter up front and have far more power!

                  What is your timing set at. I ended up using rod #1455 with a orange spring.

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                  • #10
                    Thanks for the ideas, the timing is set at stock and it is defiinately a backfire and I'll have to call eldebrock about the rods and look up the throttle blade positioning

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                    • #11
                      A true backfire is created when you loose ignition, when the spark returns you ignite the excess gas and get the backfire. Would sound somewhat like a gun being fired.

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