That is an adjustable vacuum advance made by MSD. It is not a factory one where vacuum advance is limited. Regardless, even at 12-degrees, that is far from "highest" at idle as you claimed previously.
No, you disconnect it because you get SOME advance, at idle just not fullest.
There is a lot of debate on that but Holley carbs use ported vacuum high on the metering block specifically for vacuum advance:
Yes, that's a full 30 deg advance (10 initial + 20 mechnical advance), not the 20-22 deg of vacuum advance (10 initial + 10 to 12 vacuum). Your statement that the engine have full advance at idle is wrong.
but thanks for your input...
Full vacuum advance at idle. You are now vindicated.
GM
Echlin part number. For GM FACTORY replacement vacuum advance.
P/N ID# Application Starts @ “Hg
Max Adv (Distr. Degrees @ “Hg.)
VC1837 AR11 1976 305 Blazer, Exc. Calif 6-8
12.5 @ 10.5-13.5
1976 350/400/455 Pontiac 4-bbl
VC1839 AR13 4-6
12 @ 11-13
VC1840 AR14 1975-76 350/400/455 Pontiac Firebird 6-8
10 @ 9-12
VC1841 AS3 1975-76 500 Cadillac Calif. 5-7
10 @ 13-14
VC1842 AS4 1976 350 Olds Cutlass 5-7
12 @ 13-15
VC1844 AR16 3-5
12 @ 13.5-15.5
VC1845 AS5 1978-79 425 Cadillac w/F.I. 4-6
14 @ 14-16 1977 425 Cadillac
VC1846 AR17 1977 301 Buick Skylark 3-6
13 @ 10-13 1977 301 Pontiac
VC1847 AS6 1978 403 Motor Home 4-6
12 @ 12-14
1977-79 350/403 Buick LeSabre Hi Alt, Riviera,
Olds 1977-79 350/403 Pontiac Hi Alt
VC1848 AR18 4-6
12 @ 9-12
VC1849 AR19 4-6
12 @ 7-10
VC1850 AR20 1977 350/400 Pontiac 4-6
10 @ 8-11
VC1851 AR21 1977-79 350 Buick LeSabre, Century 5-7
12 @ 11-13 1978-79 350 Pontiac
VC1852 AR22 77-78 305/350/400 Chev Truck, Heavy Duty7-9
5 @ 12-14
1975-76 350/400 Chev Truck Heavy Duty
VC1854 AR24 3-5 13 @ 10-13
VC1855 AS7 1977-79 260 Olds Cutlass 3-5
15 @ 10-12
VC1856 AR25 3-6
15 @ 10-14
VC1857 AR26 3-6
12 @ 13-1
VC1865 AR34 1973-74 350 Vette Special Hi Perf 3-5
15 @ 8.5-11.5
BTW vacuum at idle on a Buick 401, 425, 400, 430 and 455 is >14 inches. Which corresponds to FULL advance on the vacuum advance at idle.