RETURN LAND MANDATED FOR SMT AFTER FLC DISTR SMOKE IN COCKPIT.

Narrative

I WAS SERVING AS PIC/INSTRUCTOR FLYING WITH THE OWNER WHO IS NOT MULTI-ENG RATED. SMT ACFT HAD BOTH ENGS REPLACED WITH ZERO-TIME ENGS AND MAINT REQUESTED 25 HRS OP PRIOR TO 'FINE-TUNING.' PLANNED FLT WAS SQL-SCK-SAC-SQL. WE BLOCKED OUT AT PM35 LCL TIME; DID AN EXTENSIVE RUNUP; AND TOOK OFF AT PM55. ON CLB OUT; EVERYTHING WAS LOOKING GOOD. WHILE CLBING THROUGH 4000 FT; BOTH OF US THOUGHT WE SMELLED SOMETHING BURNING AND ASKED THE OTHER TO CONFIRM OUR SUSPICIONS. IT WAS AN ELECTRICAL BURNING SMELL. ABOUT 15 SECONDS LATER SMOKE BEGAN COMING OUT FROM BTWN THE TOP OF THE INST PANEL AND THE GLARE SHIELD; RIGHT BEHIND THE RPM GAUGE. 10 SECONDS AFTER THE SMOKE WAS NOTICED; IT BEGAN POURING OUT RAPIDLY. IMMEDIATELY; WE SHUT OFF THE AVIONICS PWR; BOTH ALTERNATORS AND THE BATTERY/MASTER PWR. WE BEGAN A DSCNT AND TURNED BACK TOWARDS SQL. THE SMOKE STOPPED WITHIN 10 SECONDS. (WE OPTED TO RETURN TO SQL FOR EASE OF MAINT; KNOWING IF CONDITIONS WORSENED WE COULD DIVERT TO HWD). AFTER 3 MINS; WE TURNED THE ELECTRICAL EQUIP BACK ON 1 SWITCH AT A TIME. WE DSNDED TO 2000 FT AND CALLED BAY APCH FOR TRANSITION AND TO ADVISE THEM OF OUR SIT. WE DID NOT DECLARE AN EMER. BAY CLRED US DIRECT TO SQL AND ADVISED SQL TWR WE WERE ENRTE. WE LANDED WITHOUT INCIDENT. I FORGOT TO MENTION EARLIER THAT WE ALSO DEPRESSURIZED THE CABIN DURING DSCNT TO QUICKLY CLR THE SMOKE. THE SOURCE OF THE SMOKE IS BEING INVESTIGATED.