383 intake manifold changes AFR big time

Engine, Transmission & Driveline

  1. Zwap

    Zwap Member

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    I've been posted this tread in difference places hoping to get answers that could help me...
    To make a long story short.
    This winter I change from Mopar M1 dual plane intake mainifold to the manifold that all recommended for my mild 383 .030 over, decked block, Lunati 256/262 @050 213/220, Speed Pro flat top pistons @about 0.0 in the hole, 915 heads, 650 AVS2 and Mopar HP exhaust manifold with wideband AFR meetering about 40" from the closest exhaust valve.
    All recommendations where the Edelbrock RPM intake manifold.
    So far so good.
    With the M1 IM it ran pretty good and AFR was very satified, about 14@cruise and 12-13@WOT.
    With the RPM I need to rod and jet up quite a bit to get a decent AFR@cruise.
    Well, so far so good.
    But then I floor it (and all of the information below is when I floor it), a major bog and totaly lean out then reading 16-17 AFR@WOT
    I was running .098 as main jet with M1.
    As it was way to lean my first step was to jet up to .104, not much difference in AFR.
    I then tighten the AVS one full turn, not much difference.
    I then jet up to .110, still not much difference in AFR, first bog then about 14 AFR area but stil too lean.
    Is there really that much difference in flow between the RPM IM and a "standard" dual plane IM?
     
  2. cbarge

    cbarge World Famous Barge in a Budget FCBO Gold Member

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    I recommend more fine tuning of the carb first to start.
    Had you considered changing the metering rods? Adjusting of the secondary butterfly's too soon,too late opening?? if that does not work....
    Need a Holley double pumper or a bigger carburetor maybe a 750 CFM..

    To answer your intake question,the single plane will have more flow at higher RPM's and was intended for racing-not exactly a street wise choice in my opinion.
    The RPM intake is a great street/strip intake very versatile and is between factory and the M1 intake for flow in comparison.
     
  3. Davea Lux

    Davea Lux Old Man with a Hat FCBO Gold Member

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    There is not a huge difference in the flow characteristics between those two manifolds, not enough in my opinion to cause the problems you are describing. Some things to check:

    1.) Have you checked the float levels? If the floats are set too low, the carb will run the engine lean no matter what jets you put in it.
    2.) Check the settings for the secondary air door, as if it opens too quickly, it will cause a serious bog when you mash the throttle. The bog might also be from secondary metering rods that are set too low.
    3.) Be sure the accelerator pump is working properly. You can buy a larger spray head if the problem persists to see if that helps.
    4.) Since the engine was happy with the first manifold, are you sure that there is not a gasket problem on the new one that is causing a big vacuum leak?

    Dave
     
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  4. Zwap

    Zwap Member

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    @dave,
    1.) Yes, a several times and they are @7/16 level.
    2.) It's tighten about one turn over rekommended settings.
    3.) Marine pump and oversize squirter
    4.) Well I've tried the old trick with starter gas withouth result but it could be leaking under the intake manifold @the cam side and the behaving really shows results of a vacuum leak, well there's my next step for troubleshooting.

    Thanks Dave!
     
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  5. commando1

    commando1 Old Man With a Hat on the Porch FCBO Gold Member

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    It's not the manifold. Just keep tweeking. You are on the right path.
     
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  6. Davea Lux

    Davea Lux Old Man with a Hat FCBO Gold Member

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    This won't be related to the manifold, but sometimes a brake booster failure will also cause a large vacuum leak. If your idle speed changes significantly when the brake pedal is applied, that is usually an indication of a leak inside the booster, they will often hiss all the time if leaking also.

    Dave
     
  7. CBODY67

    CBODY67 Old Man with a Hat

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    One thing that I'm thinking is that the flow capabilities of the Performer RPM might be a bit too much for the set-up you have. Same with the MP M1. That a normal Performer might be more appropriate, but that's just me.

    Going from a single plane (open plenum) to a dual plane might affect jetting a bit, but probably not much. I suspect the AVS2 is metered more for a dual plane than a single plane, though. Compare the jetting to an OEM AVS on a '70 383 4bbl (330 horsepower) carb, for a reference point. The 383/335 had a 4160 Holley that model year.

    You might also check the B/RB Mopar Performance Race Manual for carb jetting recommendations, too (usually richer than stock, a notch or so). PLUS the use of "popsicle stick" dams in a few places for better a/f flow to particular cylinders. BTAIM

    The beauty of a metering rod carb is that you can meter the light-load cruise to 14.7 (with E-zero gasoline) and then use a rod with a richer part-throttle and WOT calibrations. Change the rod, not the jet, in other words. Which is where the old Carter Strip Kit (and later Edelbrock AVS Strip Kit) came in handy. Changing just the jet shifts the whole fuel curve calibration in the direction of the jet change (richer or leaner), not just one segment of the fuel curve.

    ANY aftermarket replacement carb will probably be metered pretty good, so that it runs decently well out of the box. BUT for best performance, some tweaking for the particular application possibly could get it dialed-in closer to where it needs to be. Great that you've got an O2 sensor, for that reason.

    The bog you speak of, at what rpm are you activating WOT? Just curious.

    Make SURE that one spring is under EACH of the power pistons (which runs the metering rods up and down, with varying vacuum levels! VERY important that they be there on a metering rod carb, otherwise the rods stay in the ECO step all the time. The secondaries will have only a jet, no rods.

    Keep us posted,
    CBODY67
     
  8. Zwap

    Zwap Member

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    There is two different versions of the M1 intake manifold, mine was as I wrote earlier the dual plane version, not the single plane.
    The bog is when highway driving at about 1.800-2.000 rpm and then floor it, haven't floor it from standing still.
    I'm sure that there's one spring under both of the power pistons, now is the orange one and the power stage kick in at about 6" of vacuum, cruise and power AFR is pretty good, about 14-14.5 at cruise and about 12.5-13.2 at power.
    No drop in idle rpm when braking.
    I went from .098 to .110 main jet, to me it's a huge step in jetting just changing the intake manifold and still too lean.
     
  9. CBODY67

    CBODY67 Old Man with a Hat

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    Thanks for the update and further information. The afr seems to be pretty good.

    What is the advance curve in the distributor AND is the vacuum advance working as designed?

    What happens if you use a higher starting rpm, like 2500 or 3000rpm? When the bog happens, how long before things pickup and start to act normally? What is the afr at the WOT point just before the secondaries open, as in in "D" before the WOT kickdown detent is moved? What happened in "2" at the 2000rpm level? In "1"?

    The 6"Hg power piston spring is probably about right, as the old Holley power valves used 5.5" Hg as their vacuum setting for enrichment. Do make sure, by manually moving the power pistons that they have a full range of motion in their bores, just to make sure they do. They should kind of rattle around in their bores, as all of my OEM AVS and AFB carbs let them do.

    Just some thoughts,
    CBODY67
     
  10. Zwap

    Zwap Member

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    First of all, thanks for all the good thoughts!!

    The advance curve is 18 degree initial + 18 degree mechanical=total 36 degree, all coming in at about 2.400rpm
    About 10 degree advance at full vacuum, connected to ported vacuum.

    I'll try higher rpm to see what's happening.
    When bog happends I can slightly feel the bog but the afr shows --- wich is the leanest condition, above 17. Hard to say in time, 10th of a second to half of a second maybe, then things pick up. Enough to feel it but if I didn't have the O2 sensor maybe I hanven't notice it that much, but now as I know I feel it every time.
    I haven't try to fully open the primaries just before I floor it so I don't know the afr at that state, I'll try.
    I'll try in "2" at the 2000rpm level and in "1"
     
    Last edited: Mar 24, 2020
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  11. Zwap

    Zwap Member

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    I was at the garage tonight and a couple of things made it not possible to test the car, I hope to get rid of this things and be able to test this weekend...
     
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  12. jcslocum

    jcslocum Member

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    Larger accelerator pump jets?
     
  13. CBODY67

    CBODY67 Old Man with a Hat

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    Perhaps what you're terming "bog" is what I might call "hesitation"? The "bogs" that I have experienced last much longer than what you mention. As in "falling flat on its face" when which acceleration is desired.

    In your checks, have you tried using 12-15 degrees BTDC rather than 18?

    Thanks,
    CBODY67
     
  14. CBODY67

    CBODY67 Old Man with a Hat

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    Accel pump "shooters" diameter. Holley has bunches of them, according to style and size. Edelbrock/Carter had a few different ones. The larger ones gush more fuel quicker, as the smaller ones take the same amount (from the accel pump) and shoot a smaller stream, in a longer duration (at a possibly higher velocity).

    One more "tuning device" on a carb.
    CBODY67
     
  15. Zwap

    Zwap Member

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    Correct, hesitation it is.
    As for the pump and nozzle, at the moment I have the "marine" pump with stiffer spring and .040 nozzles mounted, stock is .031 or .035, I don't remember. Pump rod at the riches inner hole.
    I have a .043 nozzle that I can test with.
    I'll give that a try too.

    I've also ordered .113 and .116 jets to richen up at WOT
     
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  16. 70bigblockdodge

    70bigblockdodge Old Man with a Hat FCBO Gold Member

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    Air door is opening too early. Continue to tighten spring tension till it stops bogging.
    Especially if you are already pedaling it cruise.
     
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  17. CBODY67

    CBODY67 Old Man with a Hat

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    As the ONLY modulation of secondary air flow is the air valve on the secondaries, if that opens too early and/or quickly, too much air sooner than the engine needs it. By comparison, the jets you're headed toward are "more jet" than the race manual recommends, for a full drag race 440, much less a reasonably stock 383.

    As marine applications see "load" all of the time, especially on a jet boat, does that particular pump set-up have a richer calibration than the normal "car" set-up?

    The "richest" accel pump setting is not particularly making the pump "richer", as it's a fixed volume pump. The longer stroke will make it "richer" due to a longer stroke, with the output being modulated by the diameter of the accel pump shooter.

    So, with the engine stopped, position the throttle to about where you observed it to be at 2500rpm. From there, manually actuate it to WOT and watch for the accel pump's output. Reason for doing this is to see just how much pump shot is left from that primary throttle position. Don't be surprised if it's not as strong and voluminous as you perceive it might be, though. But some is better than none.

    Get the primary rod metering back to the 14.7-area (with E0 fuel). As noted, changing the jet will shift the ENTIRE fuel curve, rather than just a part of it. You hare TWO steps on the rod, "Economy" and "Power". With the car in gear, foot brake firmly applied, load the engine against the brake as you increase the rpm slowly. Watch the O2 sensor readings as the throttle is increased. When the manifold vacuum (possibly monitored with a vacuum gauge attached to a "manifold vacuum" source) drops to the stated level of the power piston spring being used, you CAN tell it when the mixture "goes richer". Easy to see. That's when you know there the power piston spring is phased correctly for when enrichment is needed. As the old Holley single-stage power valves usually opened at 5.5" Hg, your 6" Hg spring is probably about right.

    When all of the AFRs for the various vacuum levels are confirmed and somewhat "in spec", LEAVE THEM ALONE. That's NOT where the hesitation or "flat spot" is coming from. You should not need constant enrichment to cover a momentary issue. Which is where the accel pump shot might come into play. To better use what's still left in the pump at that throttle position, you might go smaller to get the pump shot to last longer rather than bigger to get a glob of fuel.

    In some respects, the current-generation of carburetors offer very good driveability (which is what the annular-discharge venturi design is about), BUT seeking to have exactly-like driveability in ALL aspects as a port-timed EFI system might be a target that is not completely obtainable. Sometimes, too, heading to "more" works worse than heading toward "less", by observation.

    Keep us posted,
    CBODY67
     
  18. Zwap

    Zwap Member

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    Thanks for all the inputs!!
    It seems to be a lot of testing this weekend (if weather permitting).
    I did some of the testing already but I will come back with all the results.
    I have full control of the rpm, vacuum and afr as the meters are close to each other in a visable panel of meters.
     
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  19. Zwap

    Zwap Member

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    I don't know if the marine pump have richer calibration but the spring is stiffer so I think it pump faster.
    Yes it's a fixed volume pump but I think that at the outer and middle hole there's not enough movement to empty the pump as much as in the inner hole, am I thinking in the wrong way perhaps?
     
  20. CBODY67

    CBODY67 Old Man with a Hat

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    The linkage position varies the total amount due to the stroke difference of the situation. The pump shooter diameter varies the duration of the shot. I know that we like to believe that richer is always better, but not in all cases, with the default mode being the OEM stock position.

    The stiffer spring might seem like a good idea, but how much stiffer is it? It might end up putting more stress on the other parts of the linkage. As long as the spring is stiff enough to push fuel through the shooter, that's all that's needed. A marine application will generally not see the on-off throttle activity that a street-driven vehicle will, not unlike a stationary irrigation motor that runs at a constant rpm most of the time.

    Enjoy!
    CBODY67