Technical Engine question

[Paidparking]

Hello,
I have a few beginner questions about my 1978 fiat 1800. After timing done at shop, new radiator, new electric ignition, my fiat would idle super low when the engine was warm and sputter out. Thise was months ago, I have since moved and havent driven it again but ran it. I removed the air filter and housing from the carb, and now it idles low when warm but didnt sputter out. Is this normal? Could it be a fuel/air problem? I am attaching a video to help out, thank you!

 

[s130]

I've never worked on a 124 but looking at your picture the carb looks like it has an idle fuel cut-off solenoid. (the little round cylinder with two wires going to it). Normally I think this should only be activated during overrun conditions. If this a being incorrectly activated then the idle fuel supply is being restricted. Temperature is also probably one of the factors governing this operation.

It could also be that the idle speed throttle plate setting is too low.

 

[124BC1]

@Paidparking

Your idle speed does seem a little low but not dramatically so. It should be possible to raise it by adjusting the idle speed screw on the carb. To locate this screw, operate the throttle/accelerator linkage, look to where the linkage joins the carb alloy body near the carb base and locate a spring loaded screw that presses against the end of one of the accelerator links/plates - turn this screw in by 1/4 - 1/2 turn and see if this raises the idle speed to what you want.

Your rev counter (tachometer) needle seems to dip below the 0 mark, this seems unusual, I wonder if someone had that tacho apart and refitted the needle incorrectly? As I said your idle speed sounds just a little low - engines obviously can't idle @ -500 rpm The tacho reading can be checked by hooking up a separate tacho e.g. some automotive multi-meters have a tacho function included.

Afaik, the idle speed cut-off solenoid valve mentioned above is open with the ignition on and closes/shuts off the idle fuel when the ignition is turned off. On older cars such as your's it's purpose was to prevent engine run-on when the engine was turned off, not cut the fuel off on the over-run. So it either works or it doesn't - your car starts and idles, therefore I don't think this solenoid valve is at fault.

As regards the idle speed sputtering out with the air filter in place but not with the filter removed?
Your speedo shows c. 60K mls (I assume the 6800 mls in your initial post is a typo?), at this mileage, I'd expect there to be a little engine wear e.g. piston rings/ cyl. bores and therefore a little blow-by gas (i.e. crankcase fumes) coming from the crankcase. This blow-by gas is fed from the engine breather on the side of the cyl/ block via a large rubber hose to the underside of the air filter unit (you can see this rubber hose in your video). These crankcase fumes are spent gases and when drawn into the engine via the air filter can cause the idle speed to drop/ cause the engine to splutter. Raising the idle speed usually solves this problem.
Essentially, what you did by removing the air filter was disconnect the engine breather hose and prevent these fumes being drawn in.

If you run the engine and place a finger over the end of the rubber hose, you'll feel pressure - this is normal. Don't worry about this, just adjust the idle speed with the air filter and breather hose connected up and it should be fine.