Vacuum leak?

[FordPrefect]

Ok, the 77 carbed car is up and running, but it's impossible to tune the carb. I can turn the mixture screw all the way in, and it seems to run better that way.

If I get the engine where I want it, putting the air filter cover back on will kill the engine. Vacuum leak?

[Odoyle]

Yes, vacuum leak is likely. Has the carb been rebuild? I fixed a persistent vacuum leak in my 32 ADFA on my 77 by replacing the 3 small rubber bushings inside the linkage throttle shafts.

[18Fiatsandcounting]

...putting the air filter cover back on will kill the engine. Vacuum leak?

Not necessarily. The idle mixture screw should be adjusted when the air filter is on and all hoses and tubes are connected. If you adjust the carb when it's "bare", it may not work right when everything is connected.

-Bryan

[seabeelt]

Check your base gasket. Often the culprit other than cracked or broken vacuum lines. ( check where they attach to the carb) I would disagree with Bryan in that the only thing connected to the carb via the air breather is the crankcase vent and the charcoal canister (if you have one). Those work off of pressure from the crankcase and intake flow vacuum at the top of the carb to recirculate those gasses. The car should run fine without either of those connected. All the other vacuum lines are typically connected at the base of the carburetor and/or intake manifold. (Unless of course I am wrong and your car is connected differently)
The car needs to be fully warmed before tuning. Drive it if you can for a good 20 minutes. Barring that let the car run until the radiator fan cycles on and off for three or four times.

When was the last time your carb was rebuilt?

[18Fiatsandcounting]

I would disagree with Bryan in that the only thing connected to the carb via the air breather is the crankcase vent and the charcoal canister (if you have one). Those work off of pressure from the crankcase and intake flow vacuum at the top of the carb to recirculate those gasses. The car should run fine without either of those connected.

I agree with you, and I was actually thinking of the final "fine tuning" of the idle mixture screw which should be done with everything connected and the car fully warmed up (as you mention). Even with a free-flow air filter, the air filter will add a slight restriction to the incoming air, and this can result in a slight richening of the mixture. Very slight, and of course the engine should run fine with or without the air cleaner.

Since the best idle seems to be when the mixture screw is turned all the way in, that sounds like it's running too rich at idle. Causes of that could be an idle jet that is too large, float level too high, or the main jets or the accelerator pump is "adding" fuel to the intake at idle.

I second the thought about rebuilding the carb if this hasn't been done.

-Bryan

[baltobernie]

+1 with Michael. Disconnect and plug all vacuum lines. Does the problem go away? If not, spray some carburetor cleaner around the base of the carb and note any change in quality or quantity of idle. CAREFUL! this stuff is flammable. Any change indicates a leak here.
I'm assuming you have ensured that all other mechanical and electrical items are working per specs. Cam timing, ignition timing, distributor advance, etc. Is the carburetor installed correctly? There must be a tiny bit of slack in the linkage; the butterflies must be completely closed at idle. The idle screw must also be set correctly. Note the photo below. If any "progression holes" are showing, the carb is allowing fuel from the secondaries and idle is not only impossible, but the mixture screw is ineffective, as well. The definitive test for this is a vacuum gauge, connected to a port below (downstream) of the butterflies. It must read zero at idle. If not, you are leaking fuel or air somewhere.

[18Fiatsandcounting]

The definitive test for this is a vacuum gauge, connected to a port below (downstream) of the butterflies. It must read zero at idle.

Do you mean the pressure would be near zero? Vacuum on an idling Fiat engine is around 17 to 21 inches of mercury (4 to 5 psi absolute) depending on cams, carb, etc. Maybe a bit less with wild cams or a racing setup.

I do agree about checking for a vacuum leak, but screwing in the idle mixture screw for the best idle seems contrary to that. I could be missing something here, though.

-Bryan

[baltobernie]

Yes, manifold vacuum would be present at idle, but none should be detected at the nipple referenced.

[Nut124]

If any "progression holes" are showing, the carb is allowing fuel from the secondaries and idle is not only impossible, but the mixture screw is ineffective, as well. The definitive test for this is a vacuum gauge, connected to a port below (downstream) of the butterflies. It must read zero at idle. If not, you are leaking fuel or air somewhere.

Bernie, aren't the progression holes supposed to be showing at idle, above the butterfly when looked from above. Once the edge of the butterfly moves above the holes, they start supplying fuel. Right?

If the butterfly is open too much at idle, the carb is running on the progression holes and the idle mixture screw becomes rather unresponsive like you state.

What do you mean with reading zero at idle below the butterflies? Zero vacuum or zero pressure? The intake vacuum is supposed to be the highest at idle and low rpm, in the 10-20in Hg range, 30in being full vacuum.

What vacuum port are you referring to?

[18Fiatsandcounting]

That's consistent with my understanding, Nut124. The way I look at it is this: The engine is basically a vacuum pump driven by gasoline. If the butterfly valve on the carb were completely closed, the vacuum in the intake manifold would be 25 inches of mercury or more. However, without any air, the engine wouldn't run, so you open the butterflies a bit and the vacuum drops to around 20 inches of Hg. At that point, the idle circuit holes in the throttle barrel are exposed to the vacuum so the idle circuit works and supplies the engine with enough gas to idle.

When you open the throttle a bit more, you expose those progression (transition) holes to vacuum and they start bleeding fuel into the intake manifold. When you open the throttle a bit more, air rushes into the carb to fill the vacuum in the intake manifold, and this air rushes past the narrowing of the venturis. According to our good friend Bernoulli, this creates a pressure drop right where the main jet opening is, and so fuel is drawn from the main jet circuit and away you go. On wide open throttle, the vacuum in the intake is near zero, that is, the pressure is close to atmospheric.

-Bryan