dgc333 said

The MP distributor has an adjustable stop for the mechanical advance built into the distributor and comes with the two light springs (pink in color) installed. The stops are set for 24 degrees of advance in the distributor. With 24 degrees in the distributor and the light pink springs your timing will all be in around 1800 rpm give or take manufacturing tolerances.

If you are running open chamber sb heads you want 34-36 degrees of total mechanical timing to be more accurate than that you would need to tune on a dyno or make many runs at the track. Closed chamber heads like the magnum head requires less total mechanical because it more efficiently mixes the fuel and air and as such requires less timing to get the maximum cylinder pressure at approx. 20 degrees ATDC thats need for max power.



As you go larger in cam duration there is more over lap and at lower rpms this results in less efficient cylinder filling which requires more initial timing to have proper lead to achieve a complete burn. Unless you have adequate initial timing it is difficult to get the idle and transition circuits in the carb.

If your only goal is going down the strip you can get away with setting total mechanical to the recommended numbers and forget about initial (lots of folks just lock out the distributor and have no timing) but for a street car you are leaving a lot of driveability on the table.

The correct way to tune an engine that has been modified from stock and is going to be driven on the street is to;

1. Dial in the initial timing using max manifold vacuum as your guide.
2. Set the mechanical in the distributor to desired total (34-36) - initial.
3. Adjust the rate of mechanical with the springs. For a street car some where in the range of 2000-2500 rpm. The springs will need to be stiffer as you remove mechanical advance from the distributor. Remember the weights are going to move less and you are starting with a higher initial.
4. Now you adjust the vacuum advance if you are using one . Remember that vacuum advance only operates at light load cruising. Again this is a condition that the cylinders are not efficiently filling and extra timing is need for a complete burn. Initial + mechanical in the distributor + vacuum advance will generally be around 50-52 degrees but seldom do you ever get in a situation that you have all of it. The key here is as much as possible at cruising manifold vacuum that does not cause detonation or throttle tip in detonation.
5. Now that you have the timing dialed in you can start fine tuning on the carb. Don't forget the carb needs to be close enough for the engine to run during the timing adjustments.

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