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  1. #1

    Default Will Carb ruin Cats?

    My 83 currently has an off-road H-pipe and no emissions control equipment (deleted smog pump, etc). I'm currently running a Demon 650 that's pretty well dialed in, but run's a bit on the rich side compared to any of my fuel injected cars. Would there be any value in adding an OEM 4-cat h-pipe or would the carb ruin them in short order?

  2. #2
    FEP Senior Member
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    simple answer. no the carb will not ruin the cats. value of doing it is your exhaust fumes will not smell as bad. the problem is the cats are meant to run on a 14 to 1 fuel mixture. hard to do with a carb. if to rich I have seen mine turn the pipes red (carb trouble , really rich). cats have been on cars since the 70's (carbs) and from time to time the car caught on fire from the cats. with FI do not hear about it any more.

  3. #3

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    That's good to hear. Would there be any value in tuning the carb to run a bit more lean? I do have a wideband installed for monitoring conditions, and currently idle around 13.7 - 14.1. This is kind of new to me as I've always been a small engine/big turbo guy and I relied heavily on the ECU for tuning.

  4. #4
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    qikgts's Avatar
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    You might want to look into modern aftermarket cats for your application that don't require air injection. Many here have done this.
    Because your smog pump is deleted, if you try and use the OE 4 cat set up it will fail.

    Great discussion in the thread below...
    http://vb.foureyedpride.com/showthre...placement-cats
    '85 GT

  5. #5

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    Quote Originally Posted by dieselmiata View Post
    Would there be any value in tuning the carb to run a bit more lean? I do have a wideband installed for monitoring conditions, and currently idle around 13.7 - 14.1. This is kind of new to me as I've always been a small engine/big turbo guy and I relied heavily on the ECU for tuning.
    Yes, but not just at idle. Your idle AFR is adjusted decent. The real carburetor tuning for efficiency (and getting rid of the stink everybody seems to fight with without "cats"), is done by tuning the idle/off-idle/transition (idle feed restriction & idle air bleed sizing) circuit, and (prior to power valve opening point vacuum acceleration (primary jets)) main circuits during all but WOT (smooth, flat 12.5:1) acceleration, where 16+:1 AFR upon transition upwards is very happily attainable, and relatively easy and inexpensive to do. Non-modified/tuned EFI and out of the box carburetors rarely achieve any such fuel efficient AFRs, because they are calibrated to safe mediocre, and universal (rich) at best. The tossed around term of "stoich" and 14.7:1 is far more about "cat" efficiency than it is about FUEL efficiency. So, don't be afraid to dig into that carburetor. The results are rewarding and can beat the pants handily off of fuel injection.

    Which Demon are you running? Perhaps it's already ready for some fine tuning inside and up top... wide band oxygen sensing is a modern convenience for analysis, but we should still make the decisions on what to do with what it tells us.
    Last edited by Walking-Tall; 07-19-2017 at 10:42 PM.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  6. #6

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    It's a road demon 650. Maybe I'm just afraid of "leaning out" the mixture based on my previous experiences with turbo cars where lean conditions are the devil. This really is my first experience tuning an carbed all-motor engine, so any advice I can get I'll take.

  7. #7

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    If it's one of these ( http://documents.holley.com/lit733.pdf ) , she's infinitely adjustable...


    Advice for a starting baseline for 1-11/16" throttle bore Demon/Holley/Similar carburetors:

    - up high IFRs (idle feed restrictions) in the new-fangled puppies (see PDF page 11) relocated to the traditional down low location, where they correctly function, submerged, unaffected by air/fuel flow instabilities (inherently increased with increased camshaft duration)
    - stock or mild engine, IFRs sized 0.031", if larger than that already (most likely)
    - above mild to wild 0.050" camshaft duration, IFRs larger, up into the neighborhood of 0.035"
    - use only #1 & #3 "emulsion" bleeds, plug numbers 2 & 4 (see PDF page 11)
    - 0.070-0.073" idle air bleeds
    - follow Idle-Eze™ instructions regarding throttle plate positions at idle, for idle speed
    - adjust mixture screws as necessary, and equally, for leanest best idle, at warmed up operating temperature


    IFRs relocated from #6 locations to #25 locations shown in the second image:



    Experiment with gradually larger, say +0.002" at a time, primary idle air bleeds (readjusting idle mixture screws after each change) until the car protests (too lean) upon light/moderate (pre main circuit and power valve enrichment) level road acceleration, then decrease back 0.002-0.004" 'til smooth as glass and brisk part throttle acceleration is had. AFRs of 16+:1 are entirely and safely possible. Your engine, oil, spark plugs, nose, and wallet will thank you. Don't fear leanness at these light/moderate levels of acceleration and load, you can't hurt anything, and the engine will tell you what it needs/wants/likes, and tell you when it's primary idle/transition circuit is too lean and when it's just right and better than ever before.

    Long story, short, any "cats" on it should be happier than pigs in $hit when dialed in... and without the smell.
    Last edited by Walking-Tall; 07-20-2017 at 09:48 PM.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

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