THE PLT HAD LEFT HIS ACFT AT LAKEPORT, CA FOR MAINTENANCE TO BE PERFORMED & BARROWED MOONEY M20E, N2571W. BEFORE THE PLT TOOK OFF, THE OWNER HEARD HIM TALKING TO SOMEONE ABOUT THE WX, BUT NO RECORD OF A WX BRIEFING WAS FOUND. WHILE EN ROUTE TO PALO ALTO, CA, THE ACFT CRASHED ON THE SIDE OF A MOUNTAIN AT AN ELEVATION OF APRX 3600 FT MSL. THE IMPACT OCCURRED ON 30 DEG DOWN SLOPING TERRAIN. AN EXAMINATION OF THE CRASH SITE REVEALED THE ACFT HAD CRASHED IN A STEEP NOSE DOWN, LEFT WING LOW ATTITUDE. SEVERAL PEOPLE HEARD THE ACFT & REPORTED THAT THE ENGINE SOUNDED NORMAL, EXCEPT IT SOUNDED LIKE THE PLT APPLIED FULL POWER JUST PRIOR TO IMPACT. WITNESSES AGREED THAT THE UPPER SLOPES OF THE MOUNTAIN WAS OBSCURED BY CLOUDS & THE BOTTOM OF THE CLOUD LAYER WAS BETWEEN 3500 & 3800 FT MSL. THE NON-INSTRUMENT RATED PLT HAD LOGGED ONLY ABOUT 7 HRS OF SIMULATED INSTRUMENT TIME.