Air:Fuel Help. Cannot Get it Rich

Discussion in 'Small Block Mopar Engine' started by dilweed, Oct 8, 2018.

  1. dilweed

    dilweed Well-Known Member

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    I’m humbly throwing in the towel and asking for some opinions/guesses on this one.
    Sorry a little long - but hopefully covering the details...
    I had been running the same fuel set-up for 4 years with no troubles. It was probably a bit undersized, but never had any issues. Fuel cell to Mallory fuel filter to Summit electric fuel pump, to Aeromotive dead head regulator. Regulated to 6psi, all 6AN line.

    The car has a 408, 10.3:1 pump gas, 850 Quickfuel carb. I shift between 6k – 6.5k. The car has been primarily more track than street over the last few years. Just a couple months ago, I’ve all of a sudden gone from always being a bit rich on the A:F to really lean. One event all is great, and the next event the car falls off about 3 tenths and a couple miles an hour. I check plugs and really lean, and now I cannot get the car rich. All plugs look uniformly lean.

    After cleaning the heck out of the carb, cleaning the fuel filter, cleaning the fuel cell, and blowing the fuel line clean, I decided to start with carb jetting ~ knowing it shouldn’t be the culprit since it has been fine for the last 4 years. I kept my 4.5 power valve in (but replaced with new), dropped the high-speed bleeds to .027 (reduction of .006), increased the PVRC jets to .057 (small increase of .002), increased the primary jets to 84 (up 8 sizes), increased the secondary jets to 92 (up 8 sizes). Performing these changes incrementally made no changes to the plugs or performance, and having ALL these changes at once also made no changes to the plugs or performance. Also new bowl and metering plate gaskets, and new gasket between carb and manifold, no cracked or missing vacuum port plugs/caps.

    With no change to anything with attempted carb tuning, I started changing fuel delivery ~ which I assumed was my problem all along. Started with checking flow of my electric pump. The pump was getting old, and sounding a little different than typical, and checking flow into a fuel jug appeared a bit slow. So out with the old and in with the new. New 10AN supply from cell to new fuel filter, 10AN from filter to new Aeromotive 11203 pump, with 8AN from pump to Mallory regulator, and 8AN return all the way back to the cell. Fuel regulated to 6psi, fuel level is middle (or slightly above) carb view window. Figured that would do it. No change to plugs or performance. I did another test of running the regulated fuel supply line into a fuel jug, and it appeared to flow fine (I think 5 gallons in 1:36). Since it is uniform across all cylinders, I really did think it is/was a supply problem.
    I thought maybe a leak below the carb like intake gasket, but it would have to be both gaskets, and leak to all cylinders. Seems unlikely. All seems normal when engine is running and I spray carb cleaner all around (where I can reach) around the manifold.

    I’m a bit stumped. Perhaps I am missing something super simple. Anyone have any experiences or words of wisdom on this? I feel like I’ve been chasing my own tail on this one.

    Thanks for any help or comments!
     
  2. j par

    j par Well-hung Member

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    :BangHead: all that for a vacuum leak!
    I call a all of a sudden Vacuum leak.
    And when you fix it remember where all that was running at before.
    I can see getting a clog in the fuel filter or getting a clog in the fuel supply or in the carburetor or a massive vacuum leak but not all of a sudden the jetting went wacko? Or all of a sudden a certain size line didn't work anymore?
    I'm reaching here but even the timing going way too advanced or something but I just don't see something running good for 4 years and then getting way out of adjustment one day. But always remember it works great till it doesn't. I think something's got to get plugged or something's got to get unplugged.
     
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    • Dubob

      Dubob Well-Known Member

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      If you are running a PCV valve and it’s stuck open it will lean you out.
       
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      • famous bob

        famous bob mopar misfit

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        I run a throttle body, and am not a carb expert, but didn`t have any trouble w/ a hot 600 horse sbc about like ur build.
        Question to those that know----------
        Wouldnt` you need to make the vents bigger to let more fuel in to richen it ??
         
      • RichB

        RichB CherryCuda65

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        A vacuum Leak! I second that cause. Spend your time looking for it.
        I had a leak thru a bolt hole in my intake manifold that I had set up for a bracket holding my throttle linkage . It took me weeks to find the leak around the bolt.
         
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        • dilweed

          dilweed Well-Known Member

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          Thanks! I'll keep looking for leaks/plugs somewhere. Puzzling, but I respect these can be a bear to chase down.
          Yeah, I wasn't terribly excited about getting a new pump and lines - but getting the pump was in the realm of necessary replacement after all these years. Besides the pump and lines, I already had everything else on hand. Still spendy nonetheless.
           
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          • dilweed

            dilweed Well-Known Member

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            Breathers, all clean and clear. Was getting some "more than normal" oil out of both of them though. That's what prompted me to clean them.
             
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            • dilweed

              dilweed Well-Known Member

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              I actually checked around all these too. I'll give it another go just in case!
               
            • j par

              j par Well-hung Member

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              Do you have any pictures? Where in Washington? Like kind of Vancouver or Seattle or Yakima? Not looking for your address mind you, but if you're somewhere close I wouldn't mind taking a look.
               
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              • j par

                j par Well-hung Member

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                Have you tried the old spray carburetor cleaner or even better yet engine starter around the carburetor a little bit and see if a vacuum leak sucks in a little bit and makes the motor rev up or die down?
                 
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                • dilweed

                  dilweed Well-Known Member

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                  I'm up near Seattle. Thanks for the offer though!
                   
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                  • dilweed

                    dilweed Well-Known Member

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                    I did, but was a bit sparing and controlled with the spray as I didn't want to shoot the carb cleaner all over the engine bay. I'll give it another go, and give it hell!
                     
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                    • n.y.340 dart

                      n.y.340 dart Well-Known Member

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                      I had an aluminum spacer that warp causing a vacuum leak. Can you try another carb? Just throwing stuff out there.
                       
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                      • j par

                        j par Well-hung Member

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                        Remember to put one of those little directional red tubes on the end. Give it a little Blast in specific areas. What kind of carburetor again?
                         
                      • j par

                        j par Well-hung Member

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                        Hey wait a minute quick fuel carb? Does that have a O2 sensor that calibrates it? Is your O2 sensor sending a bad signal to your carburetor?
                         
                      • dilweed

                        dilweed Well-Known Member

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                        Good call. I actually put another carb on there but unfortunately we got rained out this weekend. I'm hoping we can squeeze one more event in before the track closes for the season.
                         
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                        • dilweed

                          dilweed Well-Known Member

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                          Q850. No o2 sensor for calibrating. I do have an o2 gauge with sending unit down near the collector of my header. Just the one sending unit, and I don't trust the reading as I go through the traps with the gauge showing 10.5:1. No way that's accurate. It's a new sending unit too. I think I fouled my old one because it was so rich.
                           
                        • Dubob

                          Dubob Well-Known Member

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                          Just one more thought on my end. Is the vent system on fuel cell clear. Maybe on the high end it’s developing enough vacuum to restrict the fuel flow. With the car sitting maybe something got in the vent.
                           
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                          • dilweed

                            dilweed Well-Known Member

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                            Checked that as well. Vent is all clean and clear, and even small filter on it as well.
                             
                          • furrystump

                            furrystump Well-Known Member

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                            any plugs on vacuum ports, they crack with age
                             
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                            • AJ/FormS

                              AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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                              Ima thinking this on the supply side, because with amount of fuel changes you made with zero A/F changes, something shoulda got better.
                              So,How about debris lodged in the passages to the float valves, or in the regulator.
                               
                            • dilweed

                              dilweed Well-Known Member

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                              should be good here as well, but definitely going to give these another once-over.
                               
                            • dilweed

                              dilweed Well-Known Member

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                              I've pulled everything apart completely, sprayed, air hosed, etc. More than a couple times. The carb is off now, so I'm going to pull apart again and clean.
                               
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                              • AJ/FormS

                                AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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                                Before you blow,look hard, real hard. Take the float valves out and look behind them. Remove the inlet pipe, and the inlet fittings, and

                                Waitaminute
                                what are the chances you got a load of alcoholized fuel?
                                 
                              • j par

                                j par Well-hung Member

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                                I appreciate your determination and the action your taking to try and solve this.
                                 
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