When using arm in starting from fully extended position, the arm stops for 2 seconds when it reaches vertical position (kind of a float), then it kicks suddenly and continues its way in. The engine is at high idle and the arm in joystick is at full stroke. The machine is a komatsu PC340-7. Any idea?
Does it also do it while your digging or working it?
That's very common on excavators, unless it has suddenly started doing it and it did not before, I wouldn't worry about it.
That would be the correct answer to your question. It isn`t unusual for the bucket circuit to also to act the same way as gravity/weight of the arm or the bucket is playing the big reason it acts that way.
I have several excavators 3- are Komatsu and none stop or pause for a couple seconds when working, I would think something has to be wrong with your machine, unless I am misunderstanding what you are saying.
I think I have to check the arm cylinder seal and if it is clean I gonna consider it as a normal behavior
No machine should do this under load, it only happens when gravity lets the arm or bucket get ahead of the oil flow. The makeup checks in the control valve are supposed to let enough oil through to keep up, and some machines have regeneration valves which should keep the backside of the piston pressurized, but nothing's perfect. If it's something that has developed over time or just started recently I would look at piston seals first.
sounds like the arm regeneration valve...