I've received several e-mails lately about 'how to fix the loose advancers', especially on the CB750 engine. So, here's some tips:
First, avoid turning the engine with the big nut by the spark advancer when the sparkplug are in the engine. This is what causes the problem. Honda told us (shops) about this many times, but that info is lost today.
Now that yours has wallowed out the hole in the end of the crankshaft where the pin on the advancer locks into place (and perhaps the advancer was changed from being the one that originally fit that hole, also a possibility today), the advancer can slip forward or back up to 5 degrees from [whatever setting you used]. This gets annoying, at the best, when trying to set timing.
There are 2 solutions I have used. The simpler, and less common, one is used when the advancer was changed from the original, and the pin on the back of the advancer plate is smaller than the hole in the crankshaft. The fix is to go the the hobby shop or hardware store (take the advancer with you) and get a piece of brass tubing that will tightly fit on the pin. It is usually the 1/8" size (SAE) or 3mm (non-USA stuff). Then take the tube and see if it will fit into the crankshaft's hole. It might need some sanding down, in some cases, to fit. If you're fortunate, the store might have varying thickness of this tubing, which will help the process: to estimate your needed size, fit a drill bit (butt end) into the crankshaft hole until you find out what size that hole is, and select tubing OD to fit. Then you can slice off a short piece of it, fit it onto the advancer's pin, and tap the advancer into place, as it will likely be a snug fit. Then you're done.
The harder (but more common) situation involves filling in the hole when it has been made very oval by someone pulling the big nut CCW direction multiple times (for years...) to statically set the points. This was common practice on the Twins, and many wrenches tried it on the Fours, too. (Honda made the 500 and later SOHC4 bikes better able to cope with this issue...) For this one, you need some brass or aluminum sheet. I have an assortment pack from the local hardware store (0.001" to 0.010" in 0.001" steps) for this task. It involves cutting a small piece of sheet in a rectangle, then wrapping it around the pin on the advancer, and installing this tricky setup into the crankshaft. Tricks: the shim must NOT wrap all the way around the pin, because it will then not be an oval: it will be round (use tubing for that scenario instead). The shim must wrap about 270 degrees (3/4 turn) of the pin, leaving either the inside or outside of the pin bare. For your advancer, first set it in place without the locating bolt, and discern whether it will have 'slack' on the outside or inside of the hole, and wrap accordingly. I have seen both. I usually glue the shim in place when I think it is close to correct, then let it dry before installation (I use Walthers GOO, Pliobond will also work). Once in place, the advancer will be hard to remove again, maybe requiring a gear puller to get it off the crankshaft.
