The design is the visual features of a JAM MAKER shown in solid in the drawings, whether those features are of shape, configuration, ornament, or pattern, or are a combination of any of those features. Shading is used in the Figures to better illustrate the design. The broken lines shown in the drawings are for illustrative purposes and form no part of the design.Drawings of the design are included.Fig. 1 is a top front perspective view of the design;Fig. 2 is a front elevation view thereof;Fig. 3 is a right side elevation view thereof;Fig. 4 is a rear elevation view thereof;Fig. 5 is a left side elevation view thereof; andFig. 6 is a top plan view thereof.