If the zig zag feature on your 403 is not working, the cam is possibly not seated correctly or the bar that swings the needle is stuck with old oil - it turns to a hard, sticky substance like varnish.
A serious cleaning of the inside from the top, bottom and nose plate will start to alleviate the situation. Here's a picture of the bar that pushes the needlebar side to side - oil in this spot and start pushing the needlebar back and forth until it gives way and starts moving. There is about a half-inch of movement in this bar when it's working properly.
Last edited by thepolyparrot; 12-19-2011 at 01:23 PM.