[tirwin]

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You set the 5° BTDC with the 2 vacuum hoses disconnected from the dizzy. There are 2 marks Z1 (TDC) and 5°.

Rev the motor to 4k and check total timing. The vacuum will add additional timing with airflow. I want to say total timing shouldn’t exceed 28°. Factory was something like 25°. I’m going from memory.

You can get maybe 2-3° more timing from the factory spec at 4k rpms but you need to be running high octane gas and you need to be confident that everything is set up well - fuel pressures are in spec, no vac leaks, strong ignition. These motors were a bit detuned from the factory for emissions reasons.

The main thing is you want to avoid detonation with too much timing.

How timing affects performance...

When the spark plug fires the mixture of fuel and air, it creates what is called a flame front. The ignited mixture has to start and expand to cover the volume of the cylinder. This takes a relatively fixed amount of time.

As rpms increase, the time that the plug fires has to change to keep getting a good fuel burn. The plug has to fire earlier so that the most amount of fuel and air gets turned into energy.

If it fires too late or too early it is inefficient. You haven’t unlocked the most potential energy. Too early, there is not enough fuel and compression. Too late and not all the fuel burns before the exhaust valve opens which leads to unburned fuel in the exhaust (rich smelling exhaust).

Detonation (engine knock) means that instead of getting a nice even fuel burn, you get an instantaneous explosion. This results in little useful energy and damages the piston.

Fuel delivery has to change accordingly as well.

If you want to understand more read up on how the fuel and timing tables works in EFI. All the theory is the same, it’s just implemented differently with CIS, carbs, EFi...