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308 carb airbox restriction discovered

Discussion in 'Technical Q&A' started by snj5, Feb 20, 2005.

  1. hanknum

    hanknum Formula 3

    Nov 1, 2003
    2,043
    Santa Barbara
    Full Name:
    Henry
    Yeah, that's what I thought, but that is where I get the highest idle. I was always under the assumption that I should adjust the idle screws for the highest idle, is that correct?

    This is why I tried the 60 idle jets. With those I could adjust the screws down to about 3 turns, but I was loading up the plugs. Not only that, but my mpg went to something like under 10 and I did get the warning light that one of the cats was overheating. Sure signs that I was too rich, right?
     
  2. cavallo_nero

    cavallo_nero Formula 3

    Nov 3, 2003
    1,634
    colorado
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    Giovanni Pasquale
    A process I utilized with my webers on the 308 goes something like this. Pull all idle jets and blow air thru the jets and passageways. Put idle jets back in. Make sure the car is up to temp. Back off on the throttle linkage stop screws so you are running on just the idle circuit thru the progression holes. Adjust each mixture screw until max idle is achieved, then back off on that screw about a ¼ turn, then proceed to next idle screw. Then go back and double check all settings. You should be able to get enough idle just off the idle circuit and with the idle stop screws backed off all the way. Hope this helps..
     
  3. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,683
    North shore, MA
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    THE Birdman
    Henry,
    Beg, borrow or steal an LM1 gas analyzer. Take all the guesswork out. Works!! I tweaked the mixture in both of mine in a couple hours. I borrowed one from a friend locally.

    Birdman
     
  4. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
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    Russ Turner
    #554 snj5, Dec 30, 2008
    Last edited: Dec 30, 2008
    I think this is really good, but I would only back off an 1/8 of a turn. I live life on the edge.... :) Some folks would tell you if it is more than 2 or 3 turns, it's time for a different jet. That is a bit of a discussion over a pint or two of hard cider in my opinion...

    The LM-1 or -2 really does give you a great look at what is really going on and some peace of mind.

    And remember, you only do this after you are sure your ignition advance settings are dead-on.

    Don't worry about being a little lean much at off power idle as well - once the airbox and filter go on, the mixture is going to get richer as the restriction to flow is added which will functionally draw a wee bit more gas and make it a wee bit richer. And, the way these are transitioned jetted, once any throttle is applied it gets really rich really quick. With the stock muffled airbox the restriction is significant in it's effect on mixture. you'll see earlier in this thread as that restriction was removed, the carbs needed some rejetting to handle the increased flow.
     
  5. gcmerak

    gcmerak Formula 3

    Mar 17, 2008
    1,643
    Engine Bay, Georgia
    Full Name:
    George C.
    Russ,

    I just took a closer look at my airbox today. The two snorkels lead not only to an open area in the front but equally to bowed out sides of the air box as well, literally letting all but the back forth of the air filter breath freely. I still think it would be interesting to redesign the box allowing air to be pulled from both sides as well as funneling air to the oil cooler from the one side. Thought you wanted to know.

    Ciao,
    George
     
  6. hanknum

    hanknum Formula 3

    Nov 1, 2003
    2,043
    Santa Barbara
    Full Name:
    Henry
    Hey Russ,

    On your set up, you opened up the "throat" of the airbox right? If I tried doing the same without enlarging the throat, do you know what K&N part would work?
     
  7. cavallo_nero

    cavallo_nero Formula 3

    Nov 3, 2003
    1,634
    colorado
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    Giovanni Pasquale
    #557 cavallo_nero, Jan 20, 2009
    Last edited by a moderator: Sep 7, 2017
  8. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
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    Russ Turner
    Yes, I am familiar with the Merak airbox. While there is a lot to be gained by opening up both sides of the air filter in a 308 (as Cavallo has done with his hoses), I think Maserati did a good job allowing unobstructed airflow to the larger area of the filter, as opposed to the 308. This is precisely why I do not use a filter in the 308 airbox, but mount a K&N cone filter at the nose, then shape the airpath for velocity. Cavallo has opened up two more flow paths to the back and remote parts of the filter, which doesn't look so factory engineered, but I would imagine works very well. I would stick with the Maserati stock box on the Merak for the most part...unless you wanted to place two cone filters at the end of the feed hoses. :)
     
  9. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
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    Russ Turner
    I do not use the stock filter inside the airbox, but rather a K&N cone filter on the end of an extended snout simply on the front of the box. This allows max air availability and velocity at the velocity stacks, and a fully exposed element with summed suction pulses for best air flow and filtration. There is not a standard K&N that fits on the end on the stock box, but if you get the biggest cone filter they make, any shop can put an oval opening on the snout that the filter can clamp to. A tube filter in the air duct seemingly would work well also. Will post a photo tonight when I get home.
     
  10. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
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    Russ Turner
    #560 snj5, Jan 20, 2009
    Last edited by a moderator: Sep 7, 2017
    Heres the idea, then the filter sits in the side vent airflow; the "throat" was opened up when the obstructive noise absorbant fiberglass packing was removed from the snout.
    Image Unavailable, Please Login
     
  11. gcmerak

    gcmerak Formula 3

    Mar 17, 2008
    1,643
    Engine Bay, Georgia
    Full Name:
    George C.
    Bellissimo Motore!

    Ciao,
    George
     
  12. tommott77

    tommott77 Formula Junior

    Feb 1, 2009
    652
    NC
    Full Name:
    Tom
    #562 tommott77, Sep 20, 2009
    Last edited: Sep 20, 2009
    I just purchased a '79 GTS this weekend, my first Ferrari, and came across this thread searching for some cheap easy performance upgrades. Very interesting read. Just spent the better half of the afternoon reading it in it's entirety. Worth bringing back.

    Couple questions and/or ideas. It seems that most are in agreement that a setup with both individual air cleaners for each carb working in conjunction with a "gutted" air box would be the ultimate setup. It seems that this was attempted with the stock air box but the air cleaners used would not clear the top of the box.

    Now I am not sure if this is something that has come out since the original attempt or not, but there are several 1 3/4" tall air cleaners available at Pierce Manifolds now for DCNFs, the cleaners in the original attempt appear to be 2 1/2" tall. Perhaps these may fit inside the stock air box?

    http://www.piercemanifolds.com/airfilters.htm

    If there are still fitment issues with the 1 3/4" tall filters Pierce also has a filter/screen setup that filters to .009". From reading the thread it appears everyone was reluctant to try these screens due to the limited filtering capabilities. With these screens I would assume that the only elemenct that could get into the carbs would be moisutre. With the air box in place though I believe that should then become a moot point.

    Any thoughts/suggestions/or am I missing something? I am going to be about a three hour drive away from my car while it gets sorted out, took on a bit of a project, so I don't have anything in front of me to check out first hand.
     
  13. George Vosburgh

    George Vosburgh F1 Rookie

    May 26, 2011
    2,790
    Pittsburgh, PA
    I just read through this excellent thread and bringing back to the attention of the carb community. Of course I immediately went down and looked at my air box and I also have mesh and fiberglass in my '77. Something else to fix, but this entire thread is one of the best I've come across in years.
     

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