What's a good cartridge for infinity black widow?

Look for a good Sonus Blue or an ADC cartridge. I had both of them in my Black Widow, and they were magic.

In my experience Mortite dries up and is bad for that purpose. You need an ultra high compliance cartridge. Also, the Black Widow was designed for a light cartridge since the arm is easily flexed.

The chances of finding a Sonus Blue cartridge with a rubber compliance that wasn't hardened by now would be very slim. That's unfortunate as it would be ideal for the Black Widow arm(I had one in a Magnepan Unipivot I way back when). Replacement styli haven't been avaiable for those for a number of years either, Sonic Research having gone out of business long ago. Here's company that offers a variety of older cartridges: (http://www.adelcom.net/cartridge.htm)

I remember getting good results with these 'back in the day' with an Ortofon OM30 and OM40, both of which are still available. The mounting, height, weight, and compliance all worked out great for this arm.

If you can find a Sonus Blue Body...I bet soundsmith could fix it for you