B757 CREW HAD A TRACK DEV WHILE DEPARTING LGA ON THE LGA 9 SID.

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I WAS THE FO; PF. AFTER DEPARTING RWY 13 AT LGA; I EXECUTED A TURN TO 180 DEG PER THE LGA 9 DEP; WHITESTONE CLB. AT 2.5 DME OFF OF LGA VOR; I BEGAN A L TURN AS PRESCRIBED BY THE SID. THE CAPT; AS PNF; WAS MANIPULATING THE MODE CTL PANEL AND ENTERED A HDG OF 140 DEGS RATHER THAN 040 DEGS PER THE SID. I HAD ALSO ASKED HIM TO BUG ME AT A SPD OF 210 KTS PER THE SID. AS THE FLT DIRECTOR LEVELED ON THE 140 DEG HDG; I SENSED SOMETHING WAS WRONG AFTER HAVING BRIEFED THE HDGS TWICE ON THE GND. I LOOKED DOWN AT THE CHART TO VERIFY THE CORRECT HDG; BUT DUE TO THE DARKNESS; TURB; AND VERY SMALL PRINT; I WAS UNABLE TO SEE THE 040 DEG HDG. BY THE TIME ALL OF THIS HAD TRANSPIRED; ATC TOLD US TO TURN TO A 320 DEG HDG AND TIGHTEN UP THE TURN AS WE WOULD SOON BE ENTERING JFK AIRSPACE. I IMMEDIATELY BEGAN A TIGHT L TURN TO 320 DEGS AND THE REST OF THE DEP WAS UNEVENTFUL. AFTER DISCUSSING THIS SIT WITH THE CAPT LATER; WE HAD CONCLUDED THESE EVENTS OCCURRED DUE TO SEVERAL FACTORS. FIRST; THE WX PLAYED A MAJOR ROLE IN THE CONCENTRATION OF BOTH MYSELF AND THE CAPT. DUE TO THE DARK; STORMY AND TURB WX; I WAS FIXATED ON FLYING THE ACFT AND COMPLYING WITH THE DEP PROC. THE CAPT HAD BEEN DISTRACTED BY THE WX AND WAS ADJUSTING THE RADAR SO AS NOT TO FLY INTO ANY POTENTIAL CELLS THAT MAY HAVE EXISTED AND THAT DIVIDED HIS ATTN FROM MANIPULATING THE MODE CTL PANEL. THIS; IN TURN; DISTR HIM FROM LOOKING AT THE DEP CHART IN FRONT OF HIM; AND HE EXTRAPOLATED THE 140 DEG HDG FROM THE COMBINATION OF THE 180 DEG AND 040 DEG HDGS. HE ALSO STATED THAT HE HAD FLOWN A REDEYE FLT THE DAY BEFORE AND DID NOT FEEL AS 'SHARP' AS HE WOULD HAVE LIKED. SECOND; THE LGA 9 DEP CHART CONTAINS AN INORDINATE AMOUNT OF INFO WITH ROUTINGS AND DESCRIPTIONS FOR SEVERAL DIFFERENT CLBS. THEREFORE; THE PRINT USED ON THE CHART IS EXTREMELY SMALL AND IS NOT EASY TO ACCESS OR READ; ESPECIALLY IN SUCH WX AND LIGHTING CONDITIONS. SUPPLEMENTAL INFO FROM ACN 569771: I THINK IN MY MIND; THE 180 DEG HDG AND 040 DEG HDG COMBINED TO MAKE 140 DEGS.