The turn signals cancel on a timer that is based on how many feet you travel after you push the button, it counts feet instead of seconds. There is a little black box mounted to the frame a little to the rear of the speed-O, it does the job, see if it's there and hooked up to the speedometer. I'm guessing it's either not hooked up or can't be due to the type of speed-O.
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I thought that, at least on the newer bikes, the self cancel worked via a lean angle sensor. Easy to check, turn the blinker on and go straight
Well, got my service manual and finally had time to open up the bike. The '93 does come with self canceling turn signals; the module is mounted directly under the instrument panel/keyswitch between the tanks. Unfortunately, the wh/gn wire isn't connected to anything and the "new" speedometer doesn't have a reed switch to connect it to. Looks like I'll either have to get a new speedometer or else manually cancel the signals.