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Thread: Scorpion carb adjustment guide

  1. #11
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    Re: Scorpion carb adjustment guide

    Ed,

    No problem at all if dashpot is missing, would even pass cal level emission w/o it

    Barry

  2. #12
    Will
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    Re: Scorpion carb adjustment guide

    It depends on how you drive the car, Ed- missing dashpot requires you to not hop off the throttle at high revs- if you do, you can get a lean backfire. Purpose of the dashpot (if you have one) is to let vacuum increase gradually enough the motor doesn't swing full lean. If there's no pop when you snap off the throttle at 6K, I say don't worry about it. If there is, you'll wanrt to add a dashpot or see if you can compensate by enriching the mixture or tailoring your driving so you are lifting off the throttle rather than snapping off it. The latter is good practice anyway, but if you notice popping on rapid decel then your missing dashpot could be a factor. Because it's a Scorpion, there are other things that can be factors too (like air getting into the exhaust via the downpipe coupler and reigniting the exhaust mixture) so don't assume the dashpot is the only thing causing it. My.02



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    Re: Scorpion carb adjustment guide

    Problems with DMTR/DATR/DATRA carbs can usually be grouped into three catergories, vacuum leaks, idle passage issues, heavy float ... at least that's my experience in 25 years of Fiat / lancia service.

    Sometimes it's as a result of wear and tear, but often is a result of POS (previous owner syndrome)

    Vacuum leaks ... most common is a bent base plate, caused by overtightening the mounting nuts. The only surefire way to check this is carb removal, and put a straight edge across the base, straight and diagonally. Fix is relatively simple, remove all linkage assemblies and face the base on a sheet of glass with fine sandpaper and plenty of water flowing over it moving the carb in a figure 8 fashion ... main thing to watch for is not to touch the throttle blades on the sandpaper.

    The carbs also tend to wear on the primary throttle shaft, repair of this is a little more involved, as it requires removal of the primary shaft, and replacement of the teflon bushes and seals at each end of the shaft. while your at it , usually you do the one bush/seal on the secondary as well. The basic aftermarket kits don't come with these vital parts however, so you need to source a genuine Weber or Fiat/Lancia (extended) kit ... and these have the bush/seals ... the bush is actualy a strip with diagonal cut ends. When these are fitted the sometimes need to be reamed in situ. Shaft seals just prise out and push in.

    Check for primary shaft wear is grasp the linkage end of the primary, and try to move it. There will always be some movement, but if the idle speed varies when the shaft is being moved (axially) then it's definitely time to strip the carb.

    Idle passage issues can be either a blockage or an air leak.

    Blockage is straightforward ... remove all the idle parts/jets, clean with solvent and blow thru all passages with compressed air. The idle circuit has the smallest passages in the carb, so any dirt ingress tends to find its way into the idle passges and stay there. If the carb has idle passage issues, usually this can be diagnosed by looking down the primary with a flashlight while the engine is running .... if fuel can be seen dripping onto the throttle plate from the primary diffuser (the part the fuel should issue from at revs) then a blockage is almost certain.

    Most likely places are the idle jet and idle mixture screw passage.

    Idle circuit issues can also involve air leaks. If there is an ingress of air where there shouldn't be, then vacuum will not be able to draw fuel into the idle circuit from the jetwell. Usually this is the O ring on the idle mixture screw... if this is damaged / missing then air is draw down the screw threads rather than the vacuum being used to pull fuel into the passage. Other points for the ingress of air are several core plugs, fitted at the factory to block drilling holes at manufacture. These can be brass cup type plugs or simple lead shot balls hammered into the end of the passage. Diagnosing these is a little more tricky, but telltale fuel stains are the best way. Replugging them or simply using a proprietary mixture like JB weld/ Devcon to cover the offending plug usually works well.

    Float issues are also quite common in this series of carburettor. The float is made from a material called "Spansil" and over time this can become porous to the fuel it is immersed in. cCheck is quite simple, remove it and weigh it, from memory it shoud be 12.5grams. If the float is fuel soaked then it will weigh a bit more (usually around 14 grams), making it sit too low in the bowl, therefore displacing too much fuel and making the fuel level too high in the well. (same symptoms as too high a float level)

    Simple check for float soak with no weighing device is try to dig your thumbnail into the float material.... if you can see a damp spot around the mark you thumbnail has made, then the float is most likely fuel impregnated and requires replacement.

    In 25 years I have never seen the secondary throttle plate out of adjustment, as the adjustment screw is buried in its casting, and staked into place ... the only way this could be out is due to POS ... but that's definitely possible.

    SteveC

  4. #14
    Will
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    Re: Scorpion carb adjustment guide

    Steve, that's a really excellent, informative post! IMO it should be archived on here somewhere!

    A 12"x12" granite floor tile from your local tile/building supply big box is also ground extremely flat and makes a great surfacing plate, as well as lots of other uses around the shop, glued to a scrap of 3/4" ply it becomes quite durable and useful. You can mist some 3M super 77 adhesive on 600 grit carborundum paper and apply it directly to the surface plate in order to flatten carb bases, etc- then simply peel off the sandpaper and clean the granite slab with some liquid paraffin for next time. You can get surfaces nearly on par with a $5K surface grinder with a little patience. Glass as Steve mentioned is a good second choice, but it'll have to be on a hard, flat surface in order to produce a flat surface, unless you happen to have a slab of some pretty thick 1/2" or better glass laying about. In other words, don't lay a scrap of 1/8" float glass on a scrap of carpet or cardboard and expect to get a perfect flat. My.02

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    Re: Scorpion carb adjustment guide

    Will, your correct about the glass having to be thick ... I use a 300mm square of 10mm thick glass that I got from a friend who builds fish tanks for a living (which is ground flat to be optically correct for viewing the fish)... I like the granite tile idea, it would be cheaper and more robust.

    SteveC


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    Senior Lancista ecohen2's Avatar
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    Re: Scorpion carb adjustment guide

    A thousand thanks!!!

    I had a really bad cold this weekend, so I warmed up the Mighty Scorp, pulled it into the garage, got it nice and warm and then started ripping things apart in my cold induced fog.

    4 hours later, without actually doing anything other than tugging on hoses, tweaking the carb and tightening clamps my car runs better then it ever has before. No hesitation, easy starts and for the first time EVER a smooth transition between choke on to choke off.

    After reading this thread and looking at my spare carb, I was able to see where things might be going wrong and was able to make sure that they were closer to being correct. So at the end of the day I can't say what was wrong, but its fixed! Rockville auto show, HERE I COME!

    Thanks again everyone!

    Ed
    1976 Lancia Scorpion
    2004 Noble M12 GTO 3r (SOMETHING NEW!)
    http://www.lanciascorpion.com

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    Re: Scorpion carb adjustment guide

    Ed:

    let me share with you a problem I had.
    About the same symptom.

    Thought after chewing through the carb that the prob was with the carb.

    It was not!

    A small hole was in my gas hose.

    No leak!

    NO way to tell.

    The pump could not keep up with the demand for fuel.

    If you have an electric pump, it may draw so much air that it starts cavitating.

    So check the easy stuff first too:

    Hole or bad fit on the fuel inlet lead to the pump
    Check the pump for proper pressure... about 2 Lbs or so ( please correct if wrong)
    Pressure may be good flow may be bad, check flow by placing a glass jar under the hose.

    What color is that gas???? Do you have water coming out???

    Have you added anything interesting to the gas lately?

    YOu can chew the carb up looking for problems only to find that it is not your carb, it may be the fuel delivery system or contamination.

    Best of Luk.

    Fellow Scorp owner Paul van Veen

  8. #18
    Senior Lancista ecohen2's Avatar
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    Re: Scorpion carb adjustment guide

    Hmm, this is an interesting thought... I had my car running great for a long time, but now I have a problem with hot starts... I wonder if it could be related to a hole in the gas line...

    Ed
    1976 Lancia Scorpion
    2004 Noble M12 GTO 3r (SOMETHING NEW!)
    http://www.lanciascorpion.com

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