Don't know how the Petronix is affected, but the book values for the stock transistorized US ignition for 73-74 in my M110 Service Manual, for the Bosch distributor 0 231 310 002 (check the little plate on the dist) are as follows:

with vacuum at idle: 4 deg After TDC (that's what it says)
without vacuum at 1500 rpm: 13-17 deg Before TDC
at 3000 rpm: 31-35 deg Before TDC
at 4500 rpm: 37-41 deg Before TDC (if you even want to turn those rpm's unloaded...maybe just hold momentarily to see where it goes, then back off)

The vacuum advance will add about 8-12 deg of advance through the range, varying on the rpm and the vac/centrifugal combo - say 10 or so for an average "add"; for instance - your 4500 rpm 37-41 becomes approx 45-53 with vacuum; your 3000 rpm check about 39-43 vs 31-35.

If you have the slightly later 75-76 Bosch 0 231 311 001, the corresponding values are: 7A/10-16B/26-33B/29-35B,
If the 76 Bosch 0 231 309 001 (unlikely - has both adv and ret diaphragms): TDC/11-17B/26-30B/26-30B(yes)

Dwell on a mechanical points version is 30-40 deg +/-3, transistorized
With old Coil, 39-42.

You may need to work those numbers back and forth against each other, to settle in the middle of the ranges, depending on how tired your distributor's springs are. If you get unstable results, check the fit of your vacuum line or renew it.

ONE MAJOR CAUTION with these transistorized ignitions systems, at least the stock - if the coil can't fire normally, it WILL find a path to ground, and will amplify the juice until it finds a path, even if it has to make its own out the side of the amp... (Check your Petronix literature for a similar caution)
DO NOT CRANK over without normal hot plug leads, OR if you must turn engine and don't want it to fire, either disconnect the supply to the system OR ground out the primary coil distributor lead, allowing it to discharge easily.
Stan

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stanbabu@3bubbas.com
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