SBC timing tabs may be at the 2 o’clock position, or 12 o’clock (the ones I have seen look more like a vertical tube than a tab), and think there may be one other possibility.
To get started:
- Pull all the plugs, mark the #1 plug position on the distributor body (white paint, nail polish, etc.), and remove the distributor cap. Use the key to bump the starter over until you see the rotor coming around to just before the #1 Plug position.
- Now look at the balancer and find the factory line cut into it. Use your paint to put a mark on that line. If it's off toward the driver side at about 2 o'clock (with the rotor pointing near #1), then a simple bolt on timing tab will work well. If it's straight up, then you might just be able to mark the cover. Other locations will require a home-made tab.
- Buy or make a TDC tool and follow the directions to determine that the piston is at TDC. Line up your adjustable pointer so that the zero line on the tab is aligned exactly with the balancer line. Now you can always find TDC.
Put the plugs back in and the distributor cap back on.
- Start up the truck and let it warm up. Unplug vacuum line going to the distributor and plug it with a rubber plug. This means zero vacuum advance.
- Check the idle speed and set it to around 600-650 rpm in Park. This low rpm prevents mechanical advance from kicking in.
- Use a timing light to determine the current base timing. Factory recommends around 4-8 degrees BTDC, but the SBC usually runs better with at least 12. Loosen the distributor hold-down and turn it to set the timing you want. It's a lot easier if you have a dial-back timing light.
- Now increase rpm and look at how the timing advances with rpm. This is where you really need a dial-back light, because the tab markings probably go to only about 12 or 16 degrees. What you want to see is that timing starts to advance by around 1500-2000 rpm, and then by about 3000 rpm it maxes out at 18-20 degrees more than your base setting. This means it will max at about 30-32 degrees.
- let it go back to idle and then apply vacuum to the distributor vacuum advance port using a small vacuum pump or a constant vacuum line on the carburetor. With vacuum applied it should advance timing about 18-20 degrees.
- Vacuum advance will work differently when driving, depending on which carburetor port you use. If you have the line connected to full/constant vacuum it will always add 18-20 degrees, so you might have to set the curb idle lower. If its on ported vacuum it will have no advance at idle.
General guideline is to not exceed about 52-54 degrees total vacuum (base+mechanical+vacuum).
You want the vacuum advance to work because it helps fuel economy. You want the mechanical to work because it helps acceleration. Don't get confused by somebody who has a race car with locked settings and it makes great power. That's because it only operates in very limited rpm and load conditions. You want settings for a daily driver.