What was found
TAXING FOR DEPARTURE WITH NR 2 ENGINE RUNNING & APU ON, A BURNING ODOR WAS OBSERVED ON THE FLIGHT DECK. AFTER DECIDING TO RETURN TO THE GATE, THE ODOR INTENSIFIED & WAS VERY STRONG IN THE CABIN & THE FLIGHT DECK. GROUND PERSONNEL SIGNALED TO SHUTDOWN & OBSERVED SPARKS COMING FROM THE REAR OF THE ACFT. THE APU FAIL EICAS MESSAGE CAME ON AS WE WERE PERFORMING THE GROUND EVACUATION CHECKLIST. TROUBLESHOT ODOR IAW ETN-EM-72-034. NO ODORS ASSOCIATED WITH ENGINES OR PACKS. APU FAILED TO START. TROUBLESHOT APU FAIL MESSAGE IAW MANUAL. FOUND STARTER GENERATOR FAILED & APU UNDER SPEED ON CMC. ODOR DETERMINED TO BE BURNED APU STARTER GENERATOR. R & R APU STARTER GEN IAW WC 24-800-01. OPS CHECKED GOOD.