What was found
STARTING ENGINE NR 2 WAS 2 1/2 MINS TO STARTER CUTOUT WITH STRONG EXHAUST ODOR IN CABIN. NR 2 ENGINE OPERATED IAW AMM 71-00-00-712-021-N00. DURING ENGINE OPS, INSPECTION OF CABIN CARRIED OUT IAW AMM 05-51-56-940-801-C1 TO IDENTIFY ODOR. NO UNUSUAL ODORS PRESENT OR IDENTIFIED. NR 1 ENG STARTED IAW AMM 71-00-00-712-021-N00 FOLLOWED BY AN ATTEMPT TO START NR 2 ENG. N2 DID NOT REACH MAX MOTOR & STAGNATED AT 17 PSI, ENGINE START WAS THEN ABORTED. FAULT ISOLATION ACCOMPLISHED USING AMM 36-10-00/001 CONFIG 2 & TASK 36-11-05-206-001. BOTH RT & LT ISOLATION BYPASS CHK VALVES FOUND WITH DAMAGED SPRINGS & VALVE FLAPS. ADP MODULATING VALVE REPLACED IAW AMM 29-11-04/401. NO FIX. POST INSTALLATION OF MODULATING VALVE. ADP OVERSPEED SHUTDOWN TEST ACCOMPLISHED IAW AMM 29-11-00-735-116, PART P. TEST PASSED. ENGINE NR 1 RUN CARRIED OUT. NORMAL OPS AND INDICATION FOLLOWED BY NR 2 ENG RUN. NORMAL FAULT CONFIRMED TO BE THE ADP MODULATING VALVE, IDENTIFIED THROUGH THE ADP OVERSPEED SHUTDOWN TEST, & SUBSEQUENT ENGINE RUNS WITH THE NEWLY INSTALLED VALVE SHOWING NORMAL OPS AND INDICATIONS. AIRCRAFT RETURNED TO SERVICE.