We're still seeing angle valve engines (particularly the 390) with P-mags timed incorrectly. Very often, it's because the user was confused about how to clock the installation when preparing to do the timing set procedure (i.e. blow in the tube). Perhaps illustrations will help. See below.
Background: the angle valve engines require significantly less advance than the parallel valve engines, a function of their port design. Lycoming specifies 20 BTDC base timing for the 390. User testing says the maximum practical advance is 30 BTDC, and that's useful only for slow burning, very lean mixtures. For typical peak EGT or slightly lean of peak cruise, there is little practical performance difference between 25 BTDC and 30 BTDC. However, less advance almost always nets lower temperatures, with lower mechanical stress.
A P-mag's base timing, with no jumper installed between terminals 2 and 3, is 30.8 BTDC, and it can advance to 39.2....way too much. Installing the jumper dials base timing back to 26.6 BTDC, with a max of 35....still too much. Clocking the installation, as described in the P-mag manual, allows further reduction. If clocked 5 degrees (with jumper installed), the timing becomes 21.6 base and 30 max. If clocked 7 degrees with jumper, the timing is base 19.6, with a maximum of 28. The following illustrations are drawn with 5 degree clocking, mostly to stay within the recommendations in the P-mag installation manual. However, I strongly recommend 7. Just make the new timing mark 2 additional degrees to the left.
When creating a clocked timing mark, use the factory timing marks for a handy dimension reference, or count ring gear teeth. Assuming a typical 149 tooth ring gear, 5 degrees is approximately 2 teeth (2.06), and 7 degrees is just a fuzz less than 3 teeth (2.9).
Background: the angle valve engines require significantly less advance than the parallel valve engines, a function of their port design. Lycoming specifies 20 BTDC base timing for the 390. User testing says the maximum practical advance is 30 BTDC, and that's useful only for slow burning, very lean mixtures. For typical peak EGT or slightly lean of peak cruise, there is little practical performance difference between 25 BTDC and 30 BTDC. However, less advance almost always nets lower temperatures, with lower mechanical stress.
A P-mag's base timing, with no jumper installed between terminals 2 and 3, is 30.8 BTDC, and it can advance to 39.2....way too much. Installing the jumper dials base timing back to 26.6 BTDC, with a max of 35....still too much. Clocking the installation, as described in the P-mag manual, allows further reduction. If clocked 5 degrees (with jumper installed), the timing becomes 21.6 base and 30 max. If clocked 7 degrees with jumper, the timing is base 19.6, with a maximum of 28. The following illustrations are drawn with 5 degree clocking, mostly to stay within the recommendations in the P-mag installation manual. However, I strongly recommend 7. Just make the new timing mark 2 additional degrees to the left.
When creating a clocked timing mark, use the factory timing marks for a handy dimension reference, or count ring gear teeth. Assuming a typical 149 tooth ring gear, 5 degrees is approximately 2 teeth (2.06), and 7 degrees is just a fuzz less than 3 teeth (2.9).
