I had a couple of problems with my mizzen furler, and I thought I would pass along what I found. My main problem, was that the swivel that the sail attaches to and is hoisted to the top of the mast,would only rotate a little less than 360 degrees and then get stuck. I first flushed it out ,sprayed gunk, a degreaser in it, flushed more ,it did not help. Inside the interior of the furler there are two circlips that hold a approx. 5/16" x 1/2" plastic plug in place , that plugs the hole from which the ball bearings can be removed .After I removed the plug and took the ball bearings out ,I found the bearings in good shape and reinserted them. I found that without the plug the furler spun freely. Apparently the plug was entering the path of the bearings,stopping the bearings from turning. I turned the plug around and reassembled it and now the furler spins freely.
My second problem was that the gearbox was difficult to turn with the winch handle ,it turned ,but with more effort than it should take. Now I know one would assume the problem lies within the gearbox ,and I know owners have removed the bottom of the gearbox with difficulty and have even damaged the cover in the process. Here is what I found. First the interior of the gearbox can be accessed by simply unscrewing the aluminum housing that surrounds the stainless shaft in which you insert the winch handle. I put the gearbox in a vise and the shaft unscrewed easily with a monkey wrench turning the shaft This gives you plenty access to clean and inspect the gears . My gears turned freely without the shaft. The problem (friction ) was inside the shaft itself . I would have liked to remove the stainless shaft from the housing for a good cleaning ,but it did not come apart easily and not wanting to damage it ,I just sprayed gunk between the shaft and the housing ,worked it back and forth ,sprayed more ,work it more and it freed up nicely . I rinsed it out with water ,put a few drops of machine oil in, it and now rotates easily. So if the gearbox is stiff ,first unscrew the shaft to inspect the gearbox and see if the shaft itself may be your problem.
Hope this helps,