When the "set trigger" slaps the "sear bar" up, the "sear" moves downward, off the full cock notch, allowing the hammer to rotate forward. There's prolly not enuff trigger travel, to hold the "sear bar" up, far enuff, while the 1/2 cock notch rotates past the sear. Instead, the "sear bar", after being slapped, immediately returns to the down position, allowing the "sear" to dive upward into the 1/2 cock notch, stopping the hammer.
To rectify this issue, a "fly" is added to a mortice in the tumbler. The "fly" pushes the "sear" downward, as the 1/2 cock notch rotates past the "sear".
Check to see if there's a "fly" in the tumbler. If there is a "fly" in the tumbler, make sure the "fly" is tall enuff, to push the "sear" downward, far enuff to clear the 1/2 cock notch. Make sure the "fly" is free to move, and isn't stuck in it's retracted position.
Kindest Regards,
Doak
