Setting Stephenson Valves
Take the heads off both cylinders. Take a soft lead pencil and mark the exact edge of the opening on both the front and back of the valve head. You will have to move the engine to do that.
Then, with the reverse lever in neutral move the rods to the position of 12 o'clock. Take some time to make sure you have them squarely on 12. The valve must be equally located between the front and rear openings in the head. If not use the adjustment screw on the valve rod to center it. Do this on both sides of the engine. This is called "centering the valve" and is absolutely necessary to be able to properly time the engine. You will find the pencil marks very handy. Assuming your adjustment nuts never get loose you won't ever have to do this again.
Timing the Engine
1. Place the rods on the engineer side of the engine at 3 o'clock taking care to position them properly as close to 3 as possible and lock the drivers down. Put your revers lever in full forward. Move the reverse lever in full forward. Move the forward eccentric until the valve just barely covers the opening and lock it down. Then put rods in the 9 o'clock position, lock down the drivers, reverse lever in full reverse and move the reverse cam until the head covers the back opening in the head. Then lock it down.
2. Go to the fireman's side and put the rods at 9 o'clock, lock drivers down, reverse lever in full forward, move the forward cam to where the valve just covers the front opening, and lock it down. Put rods at 3 o'clock, lock down the drivers, reverse lever in full reverse and move reverse cam until the valve covers the back of the opening. Again the pencil marks come in handy.
3. Understand that moving the reverse cams will slightly affect the forward valve position and vice versa and that you will have to repeat the above steps a couple of more times to get the engine to time as close as possible.
It GREATLY simplifies things if you have one person watching the valve while another is under the engine moving the cams. They can tell you when to stop or to go the other way etc. Or, if you have not yet put the boiler on you can time it and see the valve from above.
If your cams are properly spaced 90 degrees apart and you have done everything else right while building it, you should hav enot difficulty in timing it.