"Swing" mag or fixed timing
It took me a long time to learn the reason some guys use fixed ignition timing on modified Mercs ... I'm very certain that it came from copying what they saw on alky deflectors ... copied on the theory that if it is good for padded alkie motors, it must be good for gas mod motors, too.
The reason a lot of the padded alky deflectors used fixed timing is that they were running the timing so close to top dead center there was no room to retard the spark. If you are only running 3 or 4 degrees BTDC, there is no point in letting the spark occur after TDC for any reason.
On the other hand ... on a near stock deflector ... running 30 or 40 degrees of spark advance at WOT ... it is a lot less likely to pop back and "bite" you if you can set the spark back to 10 or 15 degrees while pulling the rope.
Some racers have tried to justify running fixed timing as a real improvement by saying keeping it at a constant setting improved accelleration. If you look at the way the Merc linkage works you will see that there isn't much time that an old Merc spends on the race course with the timing other than at full advance: as you close the throttle from full to pretty much closed the timing is still against the full advance stop.