My guess is that your neighbor is correct. It won’t really smoke at idle unless it’s exceptionally rich, like running with full choke. What it does instead is gradually slow down and then stumble. When it’s lean it chuffs, kind of a small puff you can feel on the back of your hand. You can make it idle fine with the M needles by leaning it way out, but then at speed it will stumble or backfire indicating it’s too lean.
I’d guess you are quite rich at idle and the plugs are gradually fouling up. Since they are self cleaning plugs it will burn off at speed or if you rev it. It gets annoying having to keep gooseing it at lights though. A few years ago I ran with UE needles which again have the same idle diameter but are even richer at speed. Ultimately they were too rich as the plugs were coming out black.
The best tune method for me is to lean out the carbs in sequence until it chuffs, then turn it back in 1/8 turn. Do either front or back one first since you can identify the pipe it’s coming out of. When those two are good, lean out the center one and then it should chuff out both pipes.
If your fuel pump was a problem I’d expect that you’d see fuel coming out the overflow pipes and also see it pooling in the carb. My car came to me with gross jets which failed to shut off at the right time and I saw fuel in both those places. I can’t offer an opinion about the different pistons except that if the mixture is correct it should still idle.