User defined min/max values of Value Axis

You can use minimum and maximum properties of ValueAxis to set starting and ending value for your value axis.

However, those are just "suggestions". The chart will always struggle to adjust the scale of the value axis so that it displays "pretty" starting and ending values, as well as nice intermediate steps.

So if you set, say, the scale from 16 to 86. The chart most probably decide to go with the 10 and 90 instead.

Is there something I can do about it?

In a way, yes. You can increase the granularity of your value axis (by increasing its gridCount), so there are more steps and therefore smaller values can be used for start and end. Considering the above example you may do the following:

var valueAxis = new AmCharts.ValueAxis();
valueAxis.autoGridCount = false;
valueAxis.gridCount = 50;
// since we increased the number of grid lines dramatically, let's make the label display only on each 10th of them
valueAxis.labelFrequency = 10;