PWR LOSS ON TKOF. EMER LNDG ON HWY.

Narrative

I DID A PREFLT ON THE ACFT PRIOR TO TKOF. I TAXIED OUT TO RWY 30. I DID A NORMAL RUNUP AND EVERYTHING APPEARED NORMAL. AFTER TKOF ABOUT 200 FT AGL; THE ACFT ENG BEGAN TO LOSE PWR. AT THE POINT OF PARTIAL PWR LOSS; THERE WAS NOT ENOUGH RWY REMAINING TO LAND. INTERSTATE I-84 RUNS ADJACENT TO THE ARPT. I FLEW THE ACFT OVER THE FREEWAY AND LANDED IN THE WBOUND LANES; WITHOUT ANY ACCIDENT TO PERSONS OR PROPERTY. WE TAXIED THE ACFT BY POLICE ESCORT BACK TO THE BAKER ARPT. FBO EMPLOYED MECH SUSPECTED POSSIBLE MAGNETO PROBS. UPON INSPECTION; THE L-HAND MAGNETO COIL HAD BEEN ARCING ACROSS INSIDE THE MAGNETO CASE. THE MECH THEN INSPECTED THE R-HAND MAGNETO AND FOUND SOME DETERIORATION OF SOME OF THE COMPONENTS IN THE R-HAND MAGNETO ALSO. WITH THIS PARTICULAR SIT WE CAME TO THE CONCLUSION THAT THE ACFT ENG WAS NOT RECEIVING THE REQUIRED AMOUNT OF IGNITION TO SUSTAIN FULL PWR. THEREFORE CAUSING A FUEL FLOODING; MISFIRE SIT. BOTH MAGNETOS WERE REBUILT; A GND AND TEST FLT WAS PERFORMED AND CHKED OUT OK. THE MECH HAD NOTED; 'HE NEVER SEEN A MAGNETO HAVE A PROB SUCH AS THIS.' THIS ACFT IS OPERATED BOTH PART 91 AND 135; AND RECEIVES THE REQUIRED 50 AND 100 HR INSPECTIONS. I NOTIFIED THE BOISE FAA FSDO ABOUT THE INCIDENT.