SMA PLT FORGETS TO RECHK HIS GYROSCOPE PRIOR TO RADAR VECTORING AND EXPERIENCES A HDG TRACK DEV IN A NON ADHERENCE TO ATC CLRNC. PROX TO GND CLOUDS.

Narrative

I DEPARTED JEFFERSON COUNTY ARPT NEAR PORT TOWNSEND; WA; VFR. ENRTE TO BFI; WX WAS ENCOUNTERED WHICH CAUSED ME TO REQUEST IFR CLRNC TO BFI. I OBTAINED A WX BRIEFING PRIOR TO DEP FROM AN FSS BRIEFER. BFI RPTING 1700 FT OVCST; VISIBILITY OVER 5 MI. AFTER BEING GIVEN A CLRNC I WAS VECTORED (RADAR) TO THE BACK COURSE RWY 31L AT BFI. I WAS GIVEN CLRNC TO DSND FROM 4000 FT TO 2600 FT AND INTERCEPT THE LOC BACK COURSE. I DID NOT GET A NEEDLE SWING ON THE VOR INDICATOR (IT REMAINED FULLY DEFLECTED). I MAINTAINED 3600 FT FOR A BIT TRYING TO DETERMINE THE PROB. APCH THEN SAID I WAS OFF COURSE AND GAVE ME A NEW VECTOR (280 DEGS) TO INTERCEPT. AT THIS TIME I DSNDED TO 2600 FT AT WHICH TIME I HAD GND CONTACT. THE LOC NEEDLE STILL DID NOT SWING OR I MISSED SEEING IT SWING. THE DIRECTIONAL GYROSCOPE WAS THEN RECHKED (I HAD SET IT 8 MINS EARLIER) AND FOUND TO BE OFF APPROX 18 DEGS. I WAS STILL AT 2200 FT AND IN VISUAL FLT CONDITIONS. I THEN CONTACTED TWR AT BFI AND WAS GIVEN LNDG CLRNC. I BELIEVE THE DIRECTIONAL GYROSCOPE WAS OFF. I ALSO FEEL I SHOULD FLY THAT APCH WITH AN INSTRUCTOR TO VERIFY MY PERFORMANCE.