<html> <body> Geoff, This is a common problem with this style of action, and is most often caused by the combination of what has been touched on already by others (a few less likely causes are at the end of this message); - The hammer springs are weak - The hammer friction is low - The jack springs are strong - The damper springs are strong. Depending on the individual piano, there are different ways to eliminate it.  Here are some options: 1. The damper spoon timing is causing the resistance at the key to become very high at the same time that the jack spring is being compressed for letoff.  This can be helped by weakening the damper springs or changing the spoon regulation.  The test is to press the pedal and see if the double-strike still occurs.  If not, then the damper spring / spoon regulation will be the answer. For spoon lift, it just depends on the piano.  I have sometimes made the dampers lift earlier so that the spoon friction and spring resistance come earlier, so that when playing the key you automatically get over that resistance before getting to letoff.  That said, I have also made the lift later and solved the problem - so it depends on the individual piano. For damper spring tension, use your grand rep spring tool to pull back on a sample spring.  Lift the damper individually by hand and feel the resistance, then weaken the spring just enough that you can tell the difference when pulling back on the damper head with your finger.  Then check for the bobbling. 2. As others mentioned, friction is often set low on new actions to make sure they are reliable.  You can try repinning one hammer center to see if it helps, but I don't think that will make a difference, actually. This can occur in  other pianos with the same basic action design.  This design (weak hammer spring, strong jack spring) really does perform well and repeat well, but is subject to this kind of bobbling when the settings are just right / wrong (depending on your point of view!)  Maybe it is a tribute to the consistency of the spring setting processes that all of the pianos in this store are doing this! OK, 2 more possible causes that are less likely: 1. Manufacturer's sometimes change the thickness of the butt felts in production in order to change the "feel" of the letoff.  If they decide to use a thinner felt, the letoff feel is stronger (more like a grand).  If they make it thicker the letoff is smoother and power can be lower.  If you look at the jack angle as it meets the butts, you might see a difference between the actions and the action model you looked at.  This means that the bobbling is a trade-off that the manufacturer has chosen to live with in order to get the other performance levels they are looking for. 2. An unlikely but possible contributing factor is the angle of the action as it is installed in the piano.  For this to be off in multiple Yamaha pianos seems very unlikely, but if the base of the action is mounted out too far out from the strings (usually because the case parts are not fitting together right, or the keyboards are coming through with the capstans mounted in the wrong place, etc) the action will be very subject to bobbling on soft play.  Again, this isn't likely in this case - just something to keep in mind for the future. Hope this helps some! Don Mannino RPT</body> </html>