FO MAKING TKOF. UNFAMILIAR WITH ACFT. ACFT SLUED TO R; DEPARTED RWY. CAPT TOOK OVER; BROUGHT ACFT TO STOP. TAXIED BACK; MADE NORMAL TKOF.

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I WAS FLYING AN SMT X WHICH WAS CHARTERED OUT TO COVER ANOTHER COMPANY'S FREIGHT RUN. THE OTHER COMPANY'S PLT; WHO WAS AN ACQUAINTANCE OF MINE; WAS RIDING ALONG IN THE FO SEAT TO SHOW ME PARKING LOCATIONS AND THEIR COMPANY PROCS. HE WAS CURRENTLY FLYING AN SMT Y BUT WAS ALSO CAPT QUALIFIED IN AN SMT Z. AFTER TAXIING OUT FOR TKOF AT IAH; WE WERE HOLDING SHORT OF RWY 14L. I WAS GIVING THIS LEG TO THE FO AND HANDED OVER CTL OF THE ACFT AT THE HOLD SHORT LINE. WE WERE CLRED INTO POS AND HOLD MOMENTARILY; AND THEN CLRED FOR TKOF. ON THIS PARTICULAR TKOF; I SELECTED THE WATER INJECTION SYS TO THE ARMED POS. THIS SYS AUTOMATICALLY FIRES WATER INTO THE OPPOSITE ENG WHEN IT SENSES A SUDDEN REDUCTION IN TORQUE ON EITHER ENG. IT ALSO DEACTIVATES THE BLEED AIRS WHEN THE PWR LEVERS ARE INITIALLY ADVANCED AND A TORQUE RISE IS SENSED. AFTER BEING CLRED FOR TKOF THE FO ADVANCED THE PWR LEVERS. MY ATTN WAS DIVERTED INSIDE THE ACFT AS I THOUGHT IT STRANGE THAT THE BLEED AIRS HAD NOT BEEN DEACTIVATED. I REACHED OVER TO THE OTHER SIDE OF THE COCKPIT AND MANUALLY SELECTED THE BLEED AIRS OFF. AS I LOOKED BACK OUTSIDE THE ACFT; THE ACFT WAS DRIFTING R OFF THE RWY CENTERLINE. AS THE NOSE WHEEL STEERING BUTTON IS DIFFICULT TO DEPRESS ON THE L PWR LEVER FROM THE R SEAT; I WASN'T OVERLY CONCERNED AS DIRECTIONAL CTL COULD BE MAINTAINED BY RUDDER EFFECTIVENESS AS THE ACFT ACCELERATED. I TOLD THE FO TO PUSH THE NOSE STEERING BUTTON. HE REPLIED HE HAD AND THAT HE WAS USING FULL RUDDER. AT THIS POINT THE ACFT WAS INCREASING IN YAW TO THE R AND WAS HDG FOR THE SIDE OF THE RWY. I TOOK CTL OF THE ACFT AND INITIATED FULL REVERSE THRUST AND FULL BRAKING. THE ACFT DEPARTED THE RWY INTO THE GRASS AT APPROX 10 KTS. AT THAT POINT I ADDED PWR AND TAXIED BACK UP INTO POS AND HOLD ON THE RWY. THE CAUSE OF THE DEV FROM THE RWY WAS THAT THE R PROP HAD NOT COME OFF THE START LOCKS AND WAS NOT DEVELOPING ANY FORWARD THRUST. THE PROBLEM WAS COMPOUNDED BY AN AFT CENTER OF GRAVITY WHICH GIVES REDUCED NOSE STEERING EFFECTIVENESS DUE TO THE REDUCED WT ON THE NOSE TIRES. AFTER INSURING THAT THE R PROP WAS OFF THE START LOCK; I RECEIVED A NEW TKOF CLRNC AND TOOK OFF WITHOUT INCIDENT. I HAD FLOWN THIS PARTICULAR SMT X ONLY OCCASIONALLY; BUT AFTER THE INCIDENT REMEMBERED THAT THIS PARTICULAR ACFT HAD A HISTORY OF DIFFICULTY GETTING THE R PROP OFF THE START LOCK. THIS WOULD ALSO EXPLAIN WHY THE BLEED AIR REMAINED ON ON THE INITIAL TKOF; AS THE R ENG WAS NOT DEVELOPING TORQUE. THIS INCIDENT COULD HAVE BEEN AVOIDED IF I HAD FOLLOWED STANDARD PROC AND CHKED THE ENG GAUGES FOR PROPER OP PARAMETERS; INSTEAD OF LETTING MYSELF GET DISTRACTED BY A MINOR PROBLEM SUCH AS THE BLEEDS NOT DEACTIVATING. THE FO SHOULD HAVE ABORTED THE ACFT AFTER DEVIATING FROM THE CENTERLINE OF THE RWY; ESPECIALLY AFTER REALIZING HE WAS HAVING TO USE FULL DEFLECTION OF THE RUDDER. I BELIEVE COMPLACENCY PLAYED A BIG PART IN THIS EVENT; AS BOTH OF US WERE NOT ANTICIPATING ANYTHING TO GO WRONG. IN THE FUTURE; MORE PRUDENT ATTN WILL BE EXERCISED ON MY PART TO AVOID SIMILAR CIRCUMSTANCES.