Scorpion carb adjustment guide

[ecohen2]

I have the Scorpion shop manual, but does anyone have any good suggestions on how to properly adjust a stock carb for a Scorpion?

I can't seem to get mine adjusted properly, right now it either idles too low or hesitates whenever I get on the gas.

Thanks,
Ed

[Tom McGaffigan]

Ed,
The first thing you should check is if there is any air being drawn into the carb through the secondary bore of the carb. With the car running and warmed up, place your palm over the secondary opening of the carb. If the engine RPM changes then you have air leaking past the throttle plate on the secondary. You need to adjust the secondary throttle stop on the underside of the carb. See the diagram in a Weber book for its location. It is impossible to see and almost as tough to adjust. I bent a screw driver 90 degrees and was able to back off the screw thus allowing the secondary throttle to close completely. Onece this is done you can adjust the primary throttle stop to set the idle. On a Scorpion there is very little adjustment range for the mixture at idle but there is some. Connect a vacuum gauge and adjust the mixture screw until you have maximum vacuum at your desired idle RPM. If the mixture screw has little or no effect then you might want to try going up one jet size on the primary idle jet. All this assumes you have a properly timed ignition, good plugs and plug wires. If you cant stop the leaking air through the carb you are going to have to live with it of get another carb.
Tom McGaffigan

[John O]

Hey Ed,

I have my stock carb off the car and will probably not need it again for quite a while if you want to give it a try. I can bring it to the Rockville Car Show if you don't want it sooner. The only problem is that I've used the jets from it in my 34DMTR, so you'll have to swap your jets over from your stock carb - should take about 2 minutes.

John O.

[ecohen2]

Believe it or not, I actually have a spare carb that I bought from someone on this board. This is more about getting my current one adjusted properly.

I was reading the other day that with Webber carbs, if it hesitates on acceleration that means the "engine is starved for fuel." However, the article did not say if that means the fuel pump isn't getting the job done or the mixture is too lean. Any thoughts?


Tom, thanks for the info I am going to give this a try this weekend.


Ed

[Will]

Hi Ed;

Was it working before? And have you checked the obvious (vac leak in intake tract or gunk in the carb)?

Particularly with "de-smogged" Scorpions, you are likely to find a whole host of rotted vac caps and/or bits of hose with bolts shoved in the end- maybe even a golf tee ! There are a bunch of connectors on the bottom of the stock inlet manifold that you may find are open or connected to a hose and look correct until you find the other end of the hose isn't connected to anything.Don't forget to check the EGR valve.

My approach for solving an unknown carb problem is first to see if the idle picks up with propane, acetylene, etc. down the carb barrel (and you may find you can test that secondary this way too) and second, to unbolt the carb and flush it with solvents- because generally carbs don't go "out of whack" unless somebody's been monkeying with them, but cars that have been sitting are often victims of bad gas, sludge, and rotted rubber causing vac leaks. You might not NEED to clean the carb, but it doesn't hurt and IMO it's a lot easier to see what you are doing if it's off the car. Also keeps stray parts out of the inlet mani!

I have Haynes Weber book if you need short-term loaner.

[John O]

I agree (mostly).

Your car is so very, very stock that I think what is going on has more to do with age and wear than any adjustment being necessary. My experience with stock Webers is that problems occur when they are dirty or moving parts are worn, at worst PO have monkeyed with them badly.

Off the top of my head, I'd say accelerator pump - the diaphram is probably cracked/torn. With the engine off, take the cleaner top off and shine flashlight into the primary side. Then work the trottle lever a few times and you should see a pronounced squirt of gas from the arm that extends into the barrel. No squirt, no pump.

Beyond that, if you personally haven'y taken the carb off, cleaned it, and replaced all the gaskets, diaphrams, etc., do not delay, clean today!

Edit:
Also, an easy way to find vaccuum leaks (not sure you have any) is to spray carb cleaner around the carb avoiding the barrels and jets at the top and around the intake manifold. If there are no leaks, running should remain unchanged.

John O.

[ecohen2]

When I first got the car, I replaced all of the vacuum hoses with new ones. So I am pretty sure they are not leaking. As far as the hose with the bolt, I have a new bolt with a zip tie on that, so its pretty tight.

My air intake (round plastic thing on top of the carb) was cracked and I had sealed it with super glue and electrical tape. That could be leaking again, however I do have a new/used one that I can swap in today.

Ed

[John O]

Right, but you could be dealing with something as simple as a leaking gasket at the base of the carb. Ask me how I know. You won't be sure until you just go through it, do the basics, dot your i's and cross your t's.

John O.

[Will]

Your air intake shouldn't matter- it's the part(s) under vacuum you need to check- everything between the throttle shafts themselves to the intake manifold gasket. Also, " just because you fixed it yesterday don't mean it ain't breoke today", in other woirds, if you got air into the exhaust or a lean backfire, you could have popped one of those new hoses or a freshly replaced vacuum cap right off the motor. This happened to me enough times I finally vowed to weld every hole in that bitch closed the next time it comes off. :)

FWIW, the head temp sensor(s) are the same merric thread (14mm? dunno) as you'd need to plug the EGR hole, so you can smear some teflon on it and run it into the manifold to replace the EGR fitting. This has the bonus value of confusing the heck out of the next owner, who will be doubtless be confounded by what looks like an IAT sensor on a 1.8L carby.
:)

[ecohen2]

Uhg..... Discovered more of the previous owners handy work tonight....

First, the piece I always call a dashpot is missing from the carb on my car. However, I have a spare from my second carb. The difficult part is that you cannot put it on the carb without taking the carb off, I tried for hours. Is not having the dashpot going to cause a problem other than increased emissions?

Thanks as always,
Ed

[bjmarsh]

Ed,

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

Barry

[Will]

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

[FiatFactory]

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

[Will]

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

[FiatFactory]

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

[ecohen2]

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

[PAV]

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

[ecohen2]

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