Adjusting the Chuff Switch
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The chuff micro (Cherry ) switch adjustment can change with age and wear of the switch and cam lobes.
On the Big Boy the easiest way to work on the chuff switch is to drop the rear drivers to expose the switch.
Driver Removal instructions are here

Test the switch using a meter that has a continuity beeper so you can hear the switch closure as you turn the drivers.

The dwell of the closure should be about 20 degrees with 4 closures per revolution of the drivers.

To adjust you must remove the switch bracket by removing the mounting screw. It is easier if you remove the tie-wrap clamp to get access to the screw. Be sure to replace the tie-wrap to secure the wiring after the bracket is back in place and the switch is properly adjusted.

I leave the shim washers as the factory did and adjust the switch by slightly bending a arc on the switch lever  the direction depending on whether the switch is closed to long or not closed enough.

Be aware the switch is wired to the normally closed set of contacts so the cam lobes open the switch on the high side of the cam. 

Rotate the drivers and verify the switch dwell angle. It takes a very slight adjustment of the lever to get the correct adjustment. It's trial and error until it is right. I put a tiny dab of white grease on the lever when re-installing it.

This adjustment procedure can also be done to any of the new AF steam locos.