I see, thanks!
Adding keyframes on an existing path tends to generate acceleration/decceleration issues
That is the part I was missing
The only thing I could do to eliminate the acceleration was to remove the handles at the end of the acceleration and re draw the path points
I was hoping that Scene Editor's Frame Properties could be helpful, but so far I do not see any other way to remove the acceleration
I will test this further, maybe Disabling
AutoRecord before adding a new point might stop the tweening
Edit: I figured it out 
Adjusting Keyframe Time
Sometimes, in order to create a smooth function curve, it is necessary to shift the frame number of an
existing keyframe. To do this,
hold down the Control key while dragging on a selected keyframe. Drag
the mouse right to advance the keyframe to its new position, and drag left to move it back. Note that this
also affects the position of the keyframe in the other two axes
Before
.
After
.
tS76_MS_Animation.pdf
Page 26
• Tension, Bias, and Continuity: These values determine how smooth or how sharp the function
curve spline eases into or out of the specified keyframe. Note that changes in these values will
affect movement, translation, or scaling along all axes. Tension/bias/continuity can also be adjusted by double
clicking on a keyframe and using the control handles that appear (see below).
Adjusting TBC Values Visually
Right-click a keyframe and choose Curve Handles to bring up control handles that work similar to those
found in Spline Edit mode. Using these handles, you can change the shape of the curve as it enters and
exits the selected keyframe.
Changing the orientation of a handle adjusts the bias value for the selected animation parameter - in
other words, use handle direction to adjust the angle of the entry and exit trajectories for the selected keyframe.
Changing the length of the handle (by dragging the handle bar closer to or farther from the keyframe)
controls the tension value for the selected parameter. In other words, use this value to adjust how quickly
or how slowly the rate of movement, scaling, or rotation changes as frame numbers approach the selected
keyframe. Lengthening the handles means that movement, scaling, or rotation occurs at a more rapid rate
around the keyframe, while shortening the handles slows this rate.
Note: Because adjusting Tension, Bias, and Continuity values can often lead to over-dramatic variations
along other axes, it is a good idea to right-click on a keyframe to open the KF Properties panel. By clicking on
the Sharp, Smooth, or Very Smooth parameters, you can easily rectify problems that can occur
when editing TBC values.
Look Ahead Control Panel
• Bias: Determines the amount of “overshoot” when the object moves around curves.
• Tension: Determines the degree by which the object tries to follow its original orientation.
• Bank: When enabled, the animated object “leans” into curves in the motion path. The bank
amount can be specified numerically by degrees
