On 10/12/2011, Paul McCurdy wrote:
"On my existing, presumably original, vacuum advance is the code 7-14-10. The adjusting thread broke off so I bought a replacement. It has the code 6-14-8. What difference will this make"?

The three numbers are:
Vac level where it starts to move (in.-Hg)
Vac level where it finishes motion (in.-Hg)
Vac advance amount (Degrees at the distributor)

With light to moderate throttle setting the 7-14-10 will give 20 degrees vacuum advance at the crankshaft. With full throttle for full power acceleration, passing, or top speed, the vacuum advance goes away when the vacuum signal drops, and the engine runs on pre-set timing plus mechanical advance only.

The 6-14-8 vacuum unit will begin vacuum advance slightly sooner (with slightly less vacuum signal), will drop out vacuum advance slightly later (with heavy throttle), and will give only 16 degrees max vacuum advance at the crankshaft. You will most likely never be able to tell any difference in power and performance.

Vacuum advance in MGA serves two purposes. It improves throttle response just above idle speed, and it improves fuel economy at partial throttle cruising speed. When you mash the throttle for max power there is no vacuum advance (it drops out due to low vacuum signal). When setting ignition timing with a strobe light, disconnect the vacuum line. Use a set-back timing light to set timing at 32-34d BTDC at 3500 rpm (or higher) engine speed.

Original (book) spec for spark advance is:
7d static timing
12.25d mechanical advance at distributor
24.5 d mechanical advance at crankshaft
10d vacuum advance at distributor
20d vacuum advance at crankshaft

At cruising speed you will have 7+24.5+20 = 51.5d BTDC
At mid speed under max power you will have 7+24.5 = 31.5d BTDC

Modern motor fuel has generally better octane rating than when the cars were built, so they can now tolerate a bit more spark advance. It is generally acceptable to use 10 or 12d BTDC for static timing, and 34-36d BTDC at 3500 rpm (with vacuum disconnected). These engines are called High Compression, but 8.3:1 compression ratio is not particularly high my modern standards. As such, they usually run well on the lowest grade pump fuel available. If it pings at 2500 rpm with full throttle, you can try higher octane fuel, or you can retard timing a few degrees.