Idle speed on a 1968 DHSA car

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First of all - hope everyone is well and taking care of their loved ones.

I was able to take a break from work and the rain(!) and do a test today. Cold idle start up, took about 20 seconds of cranking and then caught and ran a bit lumpy at 900 rpm.

When I pressed the button to actuate the electrovalve there was little change. When I pressed and held the button down the engine RPM dropped and it stumbled and would have died if I didn’t release it. Once released the engine RPM recovered.

Is this expected behavior, or as seems likely, indicate an unwell carb?

Thanks!

[18Fiatsandcounting]

I'm trying to remember where we left off, but it seems like there are one of two possibilities:
1. When you press on that test switch to activate the electovalve, there is an air leak in the rubber hose that goes from the electrovalve to the carb, or the fast idle diaphragm on the carb (where the hose connects to) has an internal leak. For a 1968 DHSA, I'd actually be very surprised if that diaphragm didn't leak at this point. Try this: Disconnect the hose where it connects to the carb fast idle port, plug the hose with your finger, and press the switch. If the idle is still fine, then the diaphragm in the carb is the issue. If it drops, you either have a leak in the electrovalve itself or in that hose.

2. When you press on that test switch to activate the electovalve, something in the carb isn't happy with the sudden (snap) opening of the throttle. If the idle recovers after a few seconds after pressing the electrovalve, #2 is probably the issue. If it doesn't recover, #1 above is probably at play.

-Bryan

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I am shocked, I tell you shocked, that you haven’t memorized exactly where I’m at on my project. So disappointing - grin.

The hoses going to/from the electrovalves are new and well attached, so I don’t think I’ve got leaks there. I’m pretty sure the issue is with the diaphragm in the unit, and I’m not sure that those parts are readily available anymore.

Going with your suspicion that that diaphragm is bad, and if the parts aren’t readily available, can I simply blanket off and just work around it from a carb tuning point of view?

Thanks l!

[18Fiatsandcounting]

I am shocked, I tell you shocked, that you haven’t memorized exactly where I’m at on my project. So disappointing - grin.

Rest assured that I have plenty of my own Fiat issues to keep my mind full these days!

Going with your suspicion that that diaphragm is bad, and if the parts aren’t readily available, can I simply blanket off and just work around it from a carb tuning point of view?

Definitely. In fact, there are those (including me) who don't like this high idle feature and just remove it, and I'll tell you why: The engine braking when you let off the gas while you're in gear is a useful thing, and if the high idle device is working, you don't quite get as much engine braking. I think the reason for the high idle is to prevent the engine from going very lean when you close the throttle and you're still at high rpm (in gear), which leads to NOx formation. Solution: Keep the throttle slightly open under those conditions so the engine doesn't go lean, and thus the high idle circuit.

Just cap off and you'll be fine and never notice the difference.

-Bryan

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Thank you-that is exactly what I will do.