OK, I posted about this a long time ago on the old forum. But it didn't seem to sink in to anyone that has not had me do this procedure for them. Then they believe because they can hear the difference.
Now stop and think about the valve train and the rocker assemblies. On ONE side there is a push rod that opens and closes the valves. Imagine if you had a 4' board, and you rested it on a rock 1' from the end and started rocking it up and down for a couple years. Where it is pivoting on the rock it's going to wear out a groove in the board right! Well, that's the same thing that's happening in the rocker arm and the shaft. The end closest to the push rod has to carry the full load of the force. Because of that, it wears the shaft much more on that side.
See the end of the shaft here? It's worn deeper than the other end.
Now, if you adjust each valve separately, then one of them is going to be looser than the other because of the slack. See, when you put the feeler gauge under the valve closest to the push rod, it's going to take up the slop in the shaft first and then you get your adjustment. Then you do the other valve and adjust it. I bet you think you have both valves even now don't you. OK test it! No, don't start it! Test it by using the feller gauge under BOTH valves at the same time.
With it under both valves, use one finger and slide the feeler gauge out. If you were right, and the valves were adjusted properly then it should slide out evenly on both sides right! I'll bet you a dollar it doesn't!
In order to get the adjustment even under both valves you need to adjust the valve farthest away from the push rod first. Get it a bit on the snug side but not so snug that you can't slide the gauge back in when it's been pulled out all the way. Lock it down. Now for the fun part. LEAVE that end under the valve and slide the other end under the other valve and do your adjustment so it matches exactly the resistance of the first valve.
Here is a tip; If you tighten the second valve too much, the other end will slide out with no resistance because you raised it up off the valve by over tightening the second valve. THAT'S HOW YOU GET NOISY VALVES EVEN AFTER YOU DID AN ADJUSTMENT ALREADY!
The trick here is to leave the first valve adjusted and adjust the second valve until it matches perfectly. By that I mean that you should be able to slide the feeler gauge out EVENLY from the center. It may take a few tries to get this down but trust me it's worth it.
NOW if you are going to be taking your heads off at any time remember this tip. Pull both rocker shafts out and swap them to the other side. When you do this it rotates the shaft 180 degrees and gives you a smooth fresh surface to ride on. This will even out your rockers IF the rocker bushings aren't so worn out that it wouldn't make a difference.
The picture below is the same worn out shaft that's pictured above. It's just swapped to the other side and rotated 180 degrees so the rocker rides on the new surface and the notch lines up for the head bolt. This way the adjustments are as even as possible.
I hope this long winded reply makes sense to you. I have used this on several motors including motors for guys from this forum. All of them sound and run a little better as well as put a smile on the owners face.