A HS125 pilot was admonished by ATC for starting to climb to 2;000 FT just prior to WENTZ on the RUUDY 4 departure from TEB.

Narrative

[We were on the] RUUDY 4 RNAV departure out of TEB Runway 24 to cross WENTZ at 1;500; then cross TASCA at 2;000. We cross WENTZ at 1;500 and than started our climb to 2;000 just passing WENTZ. TEB Departure called and gave us a new heading and a higher altitude and said we started our climb to 2;000 just before crossing WENTZ and then gave us a frequency change. Not sure what caused the problem. We know the problem with the Newark arrivals descending over Teterboro Airport. As this is a RNAV departure; we have no other way on this SID to identify WENTZ; TASCA and RUUDY waypoints. We briefed for the departure; setting up the FMS for this SID and using VNAV. I was hand flying the airplane and at 1;500 noticed that the flight director was still calling for a climb. At WENTZ we then started our climb to 2;000. What could have caused the problem? VNAV not selected on the flight director or VNAV drop off after takeoff? We seldom do VNAV departures and in the future need to verify VNAV is selected.