between circuit miss?

Discussion in 'Fuel and Air Systems' started by Ironmike, Jun 1, 2018.

  1. Ironmike

    Ironmike Well-Known Member

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    Have a HP950 on this new motor. Good carb Ive had for years. Dyno session went great.

    so now in the car I have this annoying miss right around 1800 to 2000, only at cruising speed(very light throttle). I think its having trouble between the idle and main circuit. Give it a wee bit more throttle and its gone.

    It did not exhibit this last year on a different engine.

    Engine makes about 10 inches vac, at 1200 RPM idle, so I have 5.5 power valves in primary and secondary. Tried upping the idle air bleeds from 74 to 78, with no perceptible change. Have not changed main circuit air bleeds.

    Thought maybe a power valve change? When the miss is there, my AFR goes to an11:1'ish rich condition.

    I'm sure as hell not a carb expert and could use a suggestion or two. It's driving me nuts. With a 9 pound flywheel and 4 speed, these kind of things can be amplified and be REALLY irritating.
     
  2. Mopar Tim

    Mopar Tim FABO Gold Member FABO Gold Member

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    I think I would be looking at the power valve. JMO. No expert here....
     
  3. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    "Idle" Circuit
    WAG because I know nothing about the engine but this suggests the throttle is set too far open to begin with.
    Use the guidelines for initial timing in the "how to" section.

    Its the combination of Idle feed restriction and idle air bleeds that determines how quickly the fueling drops off. BUT, the transfer slot is also a variable restriction and air bleed. That's why getting the initial throttle position ballpark is important.
    You can certainly experiment with the current setup you have just to see the effect. But if you don't see any, now you know why; the other variables are overwhelming the effects of IAB changes. Try dropping the Primary IABs to .070 or so. It should effect both base idle and off idle. You can also just stick your finger on an IAB and see if it has effect on idle. If not, then changing them won't do any good for sure.

    This will give you a generalized idea of how air bleeds work. However with the idle circuit its a little different than shown. For idle we want it to taper off and also fuel flow at even 600 or 800 rpm is already somewhere close to the middle of the graph. Finally of course there are no idle "emulsion holes" in the Holley Idle Circuits.. Air Bleed Characteristics
     
  4. Dubob

    Dubob Well-Known Member

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    First off I am not an expert at anything. So bear with me. For discussion wouldn’t the miss fire cause your afr to go fat due to unburnt fuel? Just my train of thought that maybe it’s not the cause of the miss but a result. And I apologize for the distraction.
     
  5. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    Yes a misfire will mislead a wideband O2 sensor device into showing an incorrect AFR.
     
  6. yellow rose

    yellow rose Doctor of Thinkology.

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    If it were me, I'd look at T slot restricters and the power valve needs to be an 8.5 with your cruise vacuum.

    It could be lean waiting on the power valve. If you open the power valve sooner then it may go rich and then you can adjust that with the T slot restricters.
     
  7. yellow rose

    yellow rose Doctor of Thinkology.

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    Add to my last post...you need to measure vacuum at cruise and then set the power valve. Idle vacuum readings mean nothing to power valve opening.

    I'd bet you are 15-16 inches at a cruise. In that case you can use a 10.5 power valve.

    I still think you need some T slot restricters.
     
  8. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    I think its the other way - a lean miss, and that causes the AFR gage to briefly show 11.5 (incorrectly).

    Yes. Having a vac gage while driving is very informative.
     
  9. Ironmike

    Ironmike Well-Known Member

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    Yeah, I was thinking of running a long hose through the window to check vacuum at cruise.
    I had this same problem years ago and never nailed it down. Can't let that happen again.

    Oh yeah, distrib locked at 33 degrees. Last 3 motors I have had were run with locked out timing. No problem.
     
  10. Ironmike

    Ironmike Well-Known Member

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    Just checked a few things out this afternoon. Currently at 1200 RPM idle, my primaries are ONLY open by 1/2 turn of the idle screw. In other words if I back off the idle screw 1/2 turn, my primary blades are completely closed.

    My secondary idle adjustment is open a bit more, obviously. Thinking about backing off the secondary adjustment a lot, and opening up the primary adjustment to get the same 1150-1200 I need to idle.

    By the way, what are T slot restrictor?
    Also YR, if my cruise vac is 15 to 16 a 10.5 PV still wouldn't be open anymore than the 5.5 in there now.....or am I wrong?

    I guess I could just ditch the powervalves and up my jets 6 or 8 sizes, then see what happens.
     
  11. yellow rose

    yellow rose Doctor of Thinkology.

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    The higher number power valve will open sooner so the higher your cruise vacuum the higher the power valve opening you need. The idle vacuum setting is wrong and always has been. There is a cool video by Mark Whitner where he proves the power valve cam be open at idle and not change the A/F ratio.

    The T slot restricters are the holes that let fuel get down to the T slot in the base plate. You can thread the main body and use some brass set screws to change how much fuel you get while on the T slots.

    If you google it you can see where they go. I don't know why the aftermarket hasn't been installing them. It's just another tuning aid.
     
  12. Mattax

    Mattax Just the facts, ma'am FABO Gold Member

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    The main reason for only being open a 1/2 turn is because the timing is so far advanced. The engine will produce more torque at idle with less advance, a richer mixture, and it would allow the primary throttles to be a little more open. Then it would idle stronger with a little less rpm.
    Since that's something your probably don't want to do... yes, you can try backing off the secondaries and opening the primaries a bit.
    On older Holley's, the guidelines were .020 to .040" of transfer slot showing beneath the primary throttle blades (2 corner idle). With your HP, the slots might be longer or wider than those old Holleys, so while concept is true, the range may be a little different, maybe closer to .020" - .030"
    When the carb is off the engine, take a look at how much of that slot you can see under the throttle blades with the 1/2 turn in, and how much it changes with another 1/4 turn.

    Exactly. ^
    Power valve should open when the load on the engine gets high. When you rig the vacuum gage, you'll see opening throttle from cruise causes vacuum to drop. The more the throttles open, the lower the vacuum. At some point, more opening does not result in more acceleration or power. This is when the power valve should be open so the mixture gets richer instead of leaner. Its somewhere's between 1/2 throttle and full throttle. So if the vehicle cruises with 16"Hg, an 8.5"power valve is a much better guess than one based on idle vacuum - especially with a hot rod.

    When a carb is set up as it ought to be...
    Very large idle restrictions and other things can mess that up. But basically yes, shouldn't set PV based on idle.

    I still think its not enough fuel in the transfer slot rather than too much because IronMike wrote
    "miss right around 1800 to 2000, only at cruising speed(very light throttle)"

    That's why I suggested:
    1. Stick your finger on an IAB and see if it has effect on idle. If not, then changing them won't do any good for sure.
    2. If it passes test (1), drop the IABs, readjust idle mix if neccessary, and take it for a road test.

    And I would do this before messing with the initial throttle positions. But which ever sequence of changes you choose, do one change at time.
    ------------------------------------------------------------------------------
    The transfer slot is the "idle" circuit's main fuel outlet into the engine.
    The idle port with the needle valve (mixture screws) is below that.

    At idle, the portion of the transfer slot below the throttle blade sees vacuum and contributes fuel. The portion above the slot contributes air into the mixture.
    As the throttle opens the transfer port supplies more and more of the fuel from the idle circuit. Sometimes its useful to restrict this amount.
     
    Last edited: Jun 3, 2018
  13. Ironmike

    Ironmike Well-Known Member

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    Thanks guys! I'll do some experimenting today. Appreciate the help!
     
  14. SSG_Karg

    SSG_Karg Dismember

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    Any update?