Timing Stevenson Valve Gear can be a real challange, especially when you do it the first time. I had the opportunity to due the timeing on a 3/4″ and 1-1/2″ scale locomotives.
First, you want to set the cams. The cams consist 4 individual cams.
Next, you will do Slide Valve and Cylinder Adjustments.
1.) At rear dead center, both steam ports are closed by the slide valve
2.) With the crank pin a few degrees past read dead center, the slide valve begins to open the rear steam port, for the admission of fresh steam, and the front port, for the ejection of used steam through the exhaust port.
3.) With the crankpin at apporximately 150 – 160 degrees of rotation, the slide valve should close the rear steam port. The rest of the rotation, through the foward dead center, is performed with the help of steam expansion.
4.) The slide valve closes both steam ports as in 1.