Let me pick your brain, TT:
The Pilot circuit... If there was a plug in the pilot circuit of one (or more) of the carbs, how could this cause the issue? I mean technically. Could it be possible that a partial blockage exists that plugs up the circuit under lower vacuum but opens up under higher vacuum? The faster the engine goes, the more vacuum is present and the blockage allows enough fuel to pass that the cylinder(s) that isn't running kicks in and the engine turns faster (Idle hovers). When the engine is slowed, the vacuum pressure is not enough to overcome this blockage and the idle circuit remains closed but the engine still runs.
Does that make sense? Is it possible?
Somewhat. But, there are still some areas of misunderstanding.
The pilot circuit operates in parallel with the mains/throttle valve fuel delivery circuit.
With the slides closed or nearly closed in the carb bore air velocity is pretty low, and the venturi pressure drop is mostly negated.
The mains/throttle valve fuel delivery circuit is, for the most part, shut off. It's not a coincidence that the pilot exit port is located at the engine side rim of the slide, while the mains/throttle valve fuel delivery exit is ahead of that on the inlet air flow.
This arrangement allow the low pressure from piston fall to have the maximum effect on the pilot exit port, rather than the upstream mains/throttle valve fuel delivery circuit.
So, the pilot circuit is the "minimum requirements" fuel allotter for the engine at idle or slide mostly closed condition.
If the carbs don't have an enrichment device to dump fuel into the bore when the slides are suddenly opened (allows outside atmospheric pressure to reach all fuel exit ports), then for the engine to have enough fuel to gain rpm, the pilot mixture must be over rich. (Do recall that the fuel flow through all the fuel metering jets at a rate determined by the differential pressure across the metering orifice. The "push" comes for outside atmospheric, the differential is cause either by the piston falling or the venturi effect, or a combination of both depending on throttle, or slide position.)
So, if the pilot delivery is too lean because of fuel circuity restriction (or the lack of pressure differential on that circuit due to inlet duct changes), the engine will stumble/wheeze wherever the throttle is advanced (sudden loss o differential pressure). At above 1/4 (-ish) throttle/slide position, the piston fall pressure drop is more easily applied to the mains/throttle valve fuel delivery circuit exit, and adds more of it's delivery capability to the air inrush. At some transition point in the air velocity inrush and increasing speed curve, the venturi asserts it's contribution to the pressure drop, allowing the mains/throttle valve fuel delivery circuit to assert it's dominance.
It's rather difficult to describe operation via only written media, especially if the reader cannot form the words into mental images of carb parts, and invisible pressure points.
15 minutes at a white board would probably go a long way toward better understanding, even though I have no claim to being an artist.
I hope this helps.
Cheers,
