EMER DECLARED WHEN ENG COUGHS AFTER SWITCHING FUEL TANKS.

Narrative

ENRTE TO FTY FOR PRACTICE ILS APCHS IN IMC. HAD JUST IDENTED FTY LOC; ATL VOR AND FLANC NDB AND HAD JUST GOTTEN FTY ATIS INFO. TOLD CFII ABOARD I WAS SWITCHING FROM L MAIN TO L AUX FUEL TANKS. ABOUT 15 SECONDS AFTER SWITCHING TANKS; THE ENG SUFFERED 2 MOMENTARY PWR INTERRUPTIONS. I IMMEDIATELY DECLARED MAYDAY; MAYDAY; MAYDAY; AND ATL APCH ADVISED FTY WAS AT MY 9 O'CLOCK POS. I WAS GIVEN A HDG OF 180 DEGS. BY NOW; THE ENG WAS RUNNING FINE; BUT I DID NOT KNOW WHY WE HAD SUFFERED THE MOMENTARY PWR INTERRUPTIONS. SWITCHED BACK TO L MAIN TANKS; SINCE I KNEW IT WAS GOOD. BROKE OUT AT 2700 FT MSL. GOT A VISUAL ON FTY. CHANGED TO FTY TWR FREQ 118.5. TWR CLRED US TO LAND ON RWY 8. HOWEVER; IN ORDER TO AVOID LNDG WITH A L QUARTERING TAILWIND; I REQUESTED A CLRNC TO LAND ON RWY 32. AFTER LNDG; TAXIED TO FBO AND DISCUSSED THE SIT WITH MR. X IN MAINT AND ALSO WITH MR. Y AT FBO. SINCE THE AIRPLANE HAD JUST COME OUT OF MAINT; THE L AUX TANKS HAD BEEN COMPLETELY DRAINED. MR. Y SUSPECTED AIR IN THE LINES. MR. X AND I CAREFULLY SUMPED THE TANKS AND FOUND ONLY AVGAS. NO WATER. NO JET FUEL. DID AN EXTENSIVE RUN-UP AND RETURNED FROM FTY TO PDK. WILL HAVE ACFT CHKED BY MAINT TO VERIFY THERE IS NOTHING WRONG WITH THE FUEL LINES RUNNING FROM THE L AUX TANK. IT APPEARS TO ONLY HAVE BEEN A SLUG OF AIR.