Idle speed on a 1968 DHSA car

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It’s a good day in 124 land when I get to start fussing with my idle. Thank you for everyone’s help and my getting to this point ( I do, however, still need to pick up ignition switch power for the fuel pump but I’ll figure that out tomorrow ).

The question du jour is adjusting my idle speed. When the car is hot I am at about 3000 RPM, which suggests that I need to adjust the idle screw. What’s odd is that I did already adjust it however and it doesn’t seem to have made the difference I would’ve expected. I am assuming that counterclockwise on that screw should decrease the idle speed, is that correct?

I also discovered that I have what looks to be a choke rod that extends down from that area in a J shape which is not attached to anything. I am assuming that needs to be addressed as well - grin.

Thanks!

[wetminkey]

My gosh! 3000 rpm idle when hot?! Best shut that engine down,...!
Idle, for a manual transmission car, should not be above 800 - 900 rpm,...for my automatic transmission, idle should be about 1000.
Speed screw, turned CCW, will reduce the rpm.

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My gosh! 3000 rpm idle when hot?! Best shut that engine down,...!
Idle, for a manual transmission car, should not be above 800 - 900 rpm,...for my automatic transmission, idle should be about 1000.
Speed screw, turned CCW, will reduce the rpm.

Good morning, thanks for the response. Something is clearly not right. The idle increased with the idle speed screw being turned CCW, which is odd to put it mildly.

I am wondering if the linkage is binding somewhere....

EDIT - looking at the diagrams again I may have unscrewed the primary idle jet holder screw by mistake, that would cause the change idle. Oops!

[18Fiatsandcounting]

It sounds like you may be on the path to figuring this out. Anyway, there are 3 screws that can cause confusion with the idle circuit adjustment. All three of these screws point away from the engine. They are:
1. The fixture (screw) that hold the idle jet in the carburetor body, and it's located right below the float bowl cover and to the right of the float bowl (looking at the carb from the driver's side). [EDIT: This screw should be reasonably tight. Don't whale on it, but it shouldn't be loose. There should also be a rubber o-ring around the stem of it.] By the way, some carbs have an equivalent idle jet screw on the direct opposite side of the carb, for the secondary barrel.
2. The idle speed adjustment screw. This is located next to the primary throttle spindle linkage on the side facing the rear of the car, and it moves the throttle plate spindle ever so slightly to set the idle speed.
3. The idle mixture adjustment screw. This is located in the flange where the carb bolts to the manifold, right in the middle. Sometimes it's a screw with a spring behind it, and sometimes a screw with a locknut.

There actually is a fourth aspect to the idle, and that's the high idle circuit. That system raises the idle to around 1500 when the car is in 3rd or 4th gear and the clutch is engaged. An early emissions control device. See my later post below.

And yes, as Todd mentioned, 850 rpm is the nominal idle speed for a manual transmission Fiat of that vintage.

-Bryan

[18Fiatsandcounting]

By the way, back to your comment about the "J-hook": I believe you're referring to the high idle device I mentioned earlier. I'm not familiar with a '68 spider, but on my '69, there is a vacuum hose from the intake manifold to a round black electrovalve on the firewall across from the carb. Next to this is a small plunger type switch. Ideally, when the system is working, the electrovalve opens up and lets intake manifold vacuum to a "dashpot" on the side of the carb facing away from the engine. That dashpot is connected to a J-shaped rod" that pulls up on the throttle, thus raising the idle speed. The electrovalve is energized when you're in gear with the clutch engaged, or by pushing on that plunger (test) switch.

So, ideally, if the car is idling normally and you press that test switch, you should hear a click from the electrovalve and the idle speed should increase to around 1500 rpm. You adjust that increase with the screw that faces up in the center of that dashpot.

Hope this makes sense...

-Bryan

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Very helpful posts Bryan, thank you.

I did back off the wrong screw, oops!

Now all I need is a break in the darn rain so I can uncover the car and get this sorted!

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Good morning, should have some sunshine today to help me get this sorted.

Bryan - I loosened the idle jet holder screw by mistake, sounds like I need to snug this back up fully without over torquing it.

I will need to get a mirror to help find the idle speed adjustment screw, can't seem to find it. User error on my part I am sure.

I should probably test the electrovalve, it is there and appears to function as I can hear it actuate when I push it.

Hope everyone is doing well and staying safe.

[18Fiatsandcounting]

Bryan - I loosened the idle jet holder screw by mistake, sounds like I need to snug this back up fully without over torquing it.

I will need to get a mirror to help find the idle speed adjustment screw, can't seem to find it. User error on my part I am sure.

Sounds good on the idle jet holder screw, and yes, that idle speed adjustment screw can be hard to find and sometimes I just find it by feel (having done this many times). Assuming yours isn't missing, it is just slightly above where the primary throttle shaft sticks out of the lower body of the carb, on the side opposite the float bowl. This adjustment screw pushes on the first cam (closest to the carb body) that is mounted on the throttle shaft on that side. More "outboard" cams on this shaft are for the fast idle "J" rod, the linkage to the choke, and the "release cam" for the 2nd barrel opening mechanism.

-Bryan

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Done - idle speed set.

Reconnected the disconnected "J" rod.

Electrovalve works when I push the button, so mechanism is functioning but the car stumbles when I do this. I would have thought I'd see an RPM rise not stumble. Does that point to a mixture issue?

Thanks!

[18Fiatsandcounting]

Done - idle speed set.

Reconnected the disconnected "J" rod.

Electrovalve works when I push the button, so mechanism is functioning but the car stumbles when I do this. I would have thought I'd see an RPM rise not stumble. Does that point to a mixture issue?

Thanks!

Progress! A "stumble" when idling as you press the electrovalve button is almost certainly a vacuum leak somewhere. It could be in the hose leading from the electrovalve to the carb, or the actual dashpot mechanism in the carb itself. Bottom line: With an air leak, when you open the electrovalve and let additional air in (through the leak), you're throwing off the idle mixture which is why the engine stumbles.

The hose from the intake manifold to the electrovalve is probably OK, because this would leak all the time, but the stumble only occurs when you open that valve. My guess is the dashpot in the carb is shot. I was rebuilding a 28/36 DHSA carb just today, and the dashpot for the high idle was essentially powder. Happens pretty often on 50+ year old components.

2nd bottom line: You could live without the high idle feature. Just cap off the hose from the intake manifold.

-Bryan

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Hi there - thanks for the follow up.

The hoses to/from the electrovalve are new (I replaced them).

Carb does need a rebuild - now that I know the car is basically all there time to work the details.

I've not rebuild one of these but have several Weber IDFs (and British stuff), any tips?

[18Fiatsandcounting]

I've not rebuild one of these but have several Weber IDFs (and British stuff), any tips?

Well, the 26/34 and 28/26 DHSA are often dissed, but they are actually perfectly fine carbs, albeit with a pretty narrow primary barrel. The problem being, when the secondary barrel doesn't open as it should (very common with these carbs due to linkage friction or vacuum issues or the secondary vacuum plunger being toast), you get very low airflow into the engine and thus the performance sucks. Sucks with a capital "S".

The other problem being with the DHSA carbs is that rebuild kits nowadays don't include the secondary barrel and fast idle diaphragms. Most that I have seen just include the accelerator pump diaphragm. So, be sure to check the integrity of your secondary diaphragm. If that is shot, you may be looking at getting a new carb. You can remake the diaphragm yourself with some thin nitrile rubber sheet (I have done this), but it's not for the faint of heart.

But, other than the secondary diaphragm issue above, the rest of the carb is straightforward to rebuild. So, I'd suggest gently taking it apart and seeing what you're dealing with. Many of us on this forum have various parts and we might be able to help you out.

Or, as many on this forum have suggested, just strap on a 34 DMS or one of the ADF series like a 34ADF. The latter being one of the better carbs for a Fiat DOHC engine.

-Bryan

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Good (early) morning.

Do the other carb options bolt right up? So I need to source a different intake?

Side note - I somehow ended up with quite a few X1/9 intakes I’d anyone needs one.

[18Fiatsandcounting]

Yes, Webers like the ones I mentioned will just bolt on to your existing manifold. There can be issues with some of the various vacuum lines, such as for emission control devices or ignition advance or the like. Sometimes there are throttle linkage adjustments that need to be made. Of course, if you get into Webers like the DCOE or IDF series, those are completely different and essentially require a reworking of the intake system.

I had one thought: There is another reason that your engine might stumble when you activate the fast idle device, and that is if the accelerator pump and jet are not working right. You could test this by pulling up on that fast idle "J" linkage with your finger, and if the engine speeds up but stumbles, then it's not a leak in the electrovalve circuit that is causing that stumble. Could be the accelerator circuit, and there are also idle transition holes that smooth out the engine as it goes from idle to the main circuit. Those could be plugged.

You know, if it were me, I'd take apart your carb and clean it all up and see what actually is broken or missing before going too much further. Squirt carb cleaner through the various passages. The various plunger gaskets and seals may be just fine, with a bit of luck.

-Bryan

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Good morning and thank you.

Hopefully we will get a break in the rain and I will check that behavior and report back.

Totally agree it needs to come off and apart, just can't tackle that right now due to other moving parts. I do understand that if it is knackered I'll keep chasing my tail...

Off for more coffee now!