Thanks for all the info HM.
Is it possible to modify an advancer to work with my F? I was thinking maybe bending the tabs outward (on my "300" advancer) a bit? But then how would I measure how far to bend them?
Any of them will fit any of the bikes, and will run OK. That was one of Honda's hallmarks, which was that any later bike could accept earlier parts, and vice-versa. In a few places, this didn't work well (K0, mostly), but after K1 pretty much all can interchange, given that the whole related section goes with it.
Performance can vary, though, depending on tuning. If you put the early spark advancer in the late engine, the advance rate will be faster than the late engines' original setup. This leads to flat spots, even plug fouling problems, unless you either alter the advance rate (slow it down) or modify the carb settings accordingly. First things first: make sure the bike starts and idles well, then check the throttle-snap performance after warmup. It should be crisp, and the speed should settle down quickly when the throttle is released. If it is too slow to advance the timing, the throttle snap may feel sluggish: if too fast, the engine will back-spit and pop. Total advance on the various years changed a little (downward with time) because the US was struggling to have enough gas, and premium was downright hard to find in 1973-1975, right when the later bikes were being designed. Honda made sure the bikes up thru F1 would run on Regular, while the improved HP bikes of '77-'78 needed more octane again, as the US recovered. Still, the F2/3 engines did not have a lot of total advance, compared to the K0/K1, or even K2.
The K0/K1/K2/K3 advancers were much quicker than the post-1975 bikes that had to meet emissions tests at 2500 RPM, so those later bikes had stronger springs to delay the advance. If the faster-advance spring (or advancer) is installed in a bike with PD carbs, make sure the accelerator pump works, or it will fall on its face with a handful of throttle at 2000 RPM. This is due to spitting back into the carbs during overlap, from the waste-spark cycle being too advanced. It will then need a dose of gas to launch, which will extinguish the back-spit and wet the valve(s) again. Also, raise the needle in the slides, or increase the mainjet by a 5 size, or the bike will feel 'buzzy' as it will be lean enough to act like it is trying to ping, for the extra amount of advance it has at the lower speed. Advancing the cam timing about 3-5 degrees in the later engines will help reduce both of the problems, though, giving those bikes noticeably more midrange torque.
So, go ahead and fit it in, then you can tinker with the rest of the stuff to make it smooth. If you decide you need more total advance, you can just twist the points plate to get it, thus raising the idle timing, too. If taken too far, the engine will not throttle down crisply, which is your tip-off that the static advance is too long.
