Low Vacuum Issue/Big cam

george, easiest way to find the initial timing your engine wants is with a vacuum gauge. Keep increasing the timing in small increments while keep the rpm constant until you find the maximum manifold vacuum.

The total mecahnical is a pretty much a function of the design of the cylinder heads and you can use rule of thumb numbers to get real close. Open chamber mopar sb heads like 34-36 total. From this number subtract the initial timing you determined and that is the amount of timing you want in the distributor.

Easier than continuely stabbing the distributor back into the engine and trying is to glue a dime store protractor to an old rotor (center of protractor over center of shaft, then tape an old piece of coat hanger to the housing for a pointer. You can now twist the rotor against the advance and measure the amount in the distributor. Remember that the can turns at 1/2 the speed of the crank so the reading on the protractor needs to be 1/2 of what you would read on the crank.

Ex. if your engine wants 18 degrees initial and you shoot for 35 total you need 17 in the distributor or 8.5 when measured with a protractor.

Also, if you do a search on the web there are folks that have calculated what the slot lengths need to be for a given amount of timing.

Don't forget if the distributor is a factory stock unit the springs are likely going to be too stiff to bring the timing in at the optimal rpm. Again a good rule of thumb for a mopar sb driven on the street is to have it all in around 2500 rpm. MP sells a two spring kit that is two very light weight springs. Replace the stiffer of the two stock springs with one of these light ones and you will be very close to the 2500 rpm point.

Now the vacuum advance; You need to measure what you get for manifold vacuum while cruising on level road without a head wind. You then use a vacuum pump to actually measure what vacuum is needed to stroke the advance mechanism. There is a small allen screw inside the port on the advance mechanism that you can adjust the preload on the spring that keeps the mechanism at its zero point. Adjust so all the advance is in at your cruising manifold vacuum. If you experience detonation on throttle tip in you may need to back off on the vacuum advance to elminate it.