I recently cleaned some injectors from a 1970 280SE. I use a professional ultrasonic cleaner much like professional labs use and left them in there with some Berryman Chemstrip (I think that is what it is called) for about 24 hours. It didn't do a whole lot.

I then used Berryman carb cleaner to hose them down and scrubbed them off with a brass brush, a small one, to get the outside crud off. Then I ultrasoniced them again and the results were pretty good. You could see the little screen inside of them and they were spotless. They also had a much better spray pattern.

I guess the poor results the first time came from expecting the ultrasonic to do the heavy cleaning as well as the fine. So before you drop them in the US tank give them a good old fashioned scrubbing in the parts cleaning tank.