Hi,
First, make sure the frizzen is not actually opening all the way and bouncing back. Don't kid yourself into thinking you can see that happen. Rebound will look exactly like the frizzen is getting wedged and never opened. Before assuming the frizzen is not opening and then weakening the frizzen spring, put a piece of masking tape on the frizzen spring where the curl of the frizzen will hit when open. Fire the lock and look to see if the tape is dimpled. If the tape is dimpled, you have rebound and the frizzen spring is not too heavy, rather too light compared with the strength of the mainspring. If the tape is not dimpled, then the frizzen is not opening and you should weaken the frizzen spring a tiny bit by grinding the top leaf a little thinner. Before doing that, however, polish the top leaf and the toe of the frizzen and lubricate the spring to see if that is all you need to do. That lock has another issue. Look closely at the shoulder on the flint cock that rests on the lock plate when the cock is at rest. Notice that it is almost as deep as the edge of the plate at that point. Now imagine inletting that so the cock clears the wood. You will come close to completely exposing the edge of the plate at that point. The thick bolster part on the lock plate is too short. Rather than extend the bolster back to cover where the cock comes to rest, TOW's solution is to move the cock away from the plate. Notice how far the cock is from the plate. I weld metal on the plate, extending the bolster back until it completely covers the location where the cock comes to rest. Also notice how high the mainspring is positioned inside the lock. That almost guarantees the spring will hit the barrel unless the barrel is really skinny. You probably will have to grind the top leaf of the spring narrower to clear a barrel.
dave
