Hi Federico,
what type of transistor JFET or BJT? In any case, an hartley oscillator is not so easy to simulate accurately
because it uses several important concepts such as the resonance and the feedback.In the real circuit the
electrical noise is amplified by the tuned resonant circuit.
However, respecting certain rules of good design you can get excellent results.
For my experience if at the resonance frequency you use a resonant circuit with a
good Q factor, with an appropriate choice of inductance and capacitance, but above all, with a good biasing of the transistor, and this depends if you use a JFET or BJT, under certain conditions on the loop gain and gain of amplifier circuit imposed by the theory, you can get very good results.You can see here one of my projects.