Just make sure you ask your mechanic the right questions about your your engine is set up. And before he/she checks this, make sure they have found the true Top Dead Center for #1.
- Base timing is measured with the engine at a low idle speed (about 650-700 rpm), and the vacuum line running from the carburetor to the distributor should be disconnected at the distributor end and plugged.
On most older SBC engines the stock timing on your emissions sticker will require about 4 to 8 degrees BTDC. However if someone advanced the timing to get a little more power you may find it set to the 12 to 16 degree BTDC range.
- Mechanical advance is based on RPM, and it will add up to 18-20 degrees in addition to the base timing. On a stock engine you may start to see some mechanical timing added at about 1000 rpm and it will often max out at about 2500-3000 rpm. In other words, you put the timing light on and rev the engine up, and you will see the timing increase (the line on the balancer looks like its moving).
For most engines you don't want more than about 33-36 degrees when you add base timing to mechanical. For example, 12 degrees base and 20 mechanical would get you 32 degrees total (which should work fine). For emissions inspection purposes you might have to set it at 4 degrees base and end up with only 22-24 degrees total timing.
- Vacuum advance is only there to increase your fuel economy. When your engine is cruising on the highway the carburetor vacuum is high and it "pulls" vacuum on the vacuum line running to the distributor and moves the vacuum advance. This will usually advance your timing by about 18-20 degrees, which helps your gas mileage. That means your cruise timing could be 12 base + 18 mechanical + 18 vacuum, for a total of 48. In general, you don't want to go any higher than about 52-54, but this is not set in stone. If you increase base and mechanical to get better performance you might get pinging at light throttle when cruising. That means you have to buy an adjustable vacuum advance so you can set it lower and avoid the pinging at light throttle.