CTLR AT LGA ATTEMPTS TO ESTABLISH 2 ACFT IN-TRAIL FOR RWY 31. SEPARATION WAS LOST WHEN THE CTLR MISJUDGED SPD AND RATE OF TURN OF THE LEAD ACFT.

Narrative

I WAS WORKING FINAL VECTOR POS AT NY TRACON (LGA AREA). THE APCH IN USE WAS EXPRESSWAY VISUAL RWY 31. ACFT ARE VECTORED TO THE LGA 225 DEG RADIAL TO A POINT 5 DME SW OF LGA FOR A R TURN TO DOWNWIND AND BASE AND LAND RWY 31. ACFT A (B727) IS LEVEL AT 3000 FT 8-10 DME SW OF LGA; ON LGA 225 DEG. ACFT B (GULF) LEVEL AT 4000 FT SWBOUND 2-3 DME FROM LGA. I PLAN TO VECTOR ACFT B TO THE DOWNWIND IN FRONT OF ACFT A. I REDUCE ACFT A'S AIRSPD. ACFT B IS FIRST TURNED L HDG 250 DEGS AND DSCNT TO 3000 FT; THEN 100 DEGS TURN TO L AND ISSUED TFC AT 1 O'CLOCK POS AND 4 MI. ACFT A IS ISSUED L TURN 350 DEGS AND ISSUED TFC. ACFT B IS TURNED AGAIN; L 080 DEGS. ACFT A TURNED FURTHER L TO 320 DEGS; ISSUED TFC AGAIN. AT THIS TIME ACFT A IS APPROX 2 NM FROM ACFT B ON DIVERGING HEADINGS. ACFT A (B727) ADVISES HE SEES ACFT B. I ASK ACFT A TO MAINTAIN VISUAL SEPARATION. ACFT B (GULF) AIRSPD IS TOO FAST FOR THE TURN AND A STRONG NW WIND DID NOT HELP IN THIS SIT. PLTS THAT FLY THIS APCH; MOST TIMES MAKE THIS TURN WITHIN 3-4 MI RADIUS OF LGA VOR. THIS ACFT'S TURN WAS 6-7 MI RADIUS; MUCH WIDER THAN NORMAL. THE OTHER FACTOR IN THIS IS; NORMALLY I DO NOT HAVE ACFT AT SAME ALT; HEAD ON. I SAW A HOLE AND THOUGHT I COULD FIT THE GULF INTO THIS SO CALLED GAP.