The results of medium resolution (R=600) spectrophotometry of 15 bright carbon stars of class N, obtained in the range of lambda:1 to 4.2 microns, are presented. Data is complemented by wide band photometry in JHKLM for a bigger sample. The selected sample of stars does not show evidence of massive circumstellar dust envelopes. The infrared flux is expected to arise from a little disturbed photosphere. The observed flux distributions are compared with those predicted by model atmospheres. The presence, or not, of polyatomic molecules is taken into account. The effective temperatures of the stars is calculated. Important differences arise between the observed spectra and the predictions of models. Their validity, and the possible origins of the disagreement, are discussed.