The only mechanical advance I know of on the GL1100 is at the rear of the engine. There is the vacuum advance and underneath it there is the centrifugal advance.
You can see the diaphragm housing on the top rear right of the back cover plate. There are three screws that hold the cover on the advance. An application of a bit of vacuum on the vacuum diaphragm with the cover off will show if the vacuum diaphragm can move the pickup coil mounting plate. With no vacuum it should be in it's most counterclockwise position. With some applied vacuum it should move clockwise. On the bike I'm currently working on the backing plate was stuck and required some exercising and a bit of lube free things up.
If you remove the vacuum advance housing you'll find the centrifugal advance. If that's sticking it will keep the spark too advanced and make the starting hard.
One thing I noticed when in there is that the right hand pulser coil is pretty close to the wall of the housing and the coil wire was rubbing on the wall. It turned out that the wire had slipped out of the little clip just below the coil and the slack was allowing the wire to touch the wall. Pulling the wire down carefully, inserting it back into the clip and judiciously squeezing the clip a bit got the necessary clearance to prevent wearing the insulation through.