'05 America, AI removed, ProCom CDI set on #4, Iridium plugs, Stock mufflers w/Four 1/2-inch holes drilled into each end baffle, 135 Mains, 42 Pilots (measured to be .017" diameter), Screws 1-1/2 turns out, ZBAD needles, 3200' above sea level.
Warmed-up, Synch within 1/2 inch on both tubes; however, @3500 rpm (where I cruise at), the left carb is 4-inches higher on the manometer.
Set Synch to be within 1/2-inch @3500 rpm; now idle synch shows left side 6-1/2 inches higher. Moving right air screw out 1/2 turn brings the synch a little closer: 5-1/2 inches difference, this is as close as it gets.
No expert by any stretch - but my understanding is that carbs get synchronized at the idle setting only [ throttle plates are closed]. This sets the break open timing to match each throat. Once the throttle plate opens then the amount of air flow through the port is so large slight variance is insignificant. Just my understanding ?
I'm thinking that the manometer is measuring how strong is the vacuum signal: how strong each particular cylinder is operating. Since the operator (me) spends most of his riding time with the motor @ 3500 rpm, I have attempted to optimize the workings of each cylinder at that speed. I admit that I have not addressed the air/fuel ratio yet. Since I live in rural America, my riding is much more 'Go' than "stop and go".
Yes, that is exactly what you are measuring. Manifold vacuum. You are just synchronizing the two inlets to be the same vacuum. I use dial vacuum gauges over manometers. Just personal preference. at idle my Bonneville is at 7 in. vac. @ each inlet.
290-320 hpa (8.5 to 9.44 inHg) is typical for 270º engines like the OP's. The 360º twins get a bit higher vacuum readings, something like 360hPa (11 inHg).
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