I know, I promised this 2 years ago, before I was enshrined in the Thoughts of Hondaman thingy...maybe this should be added to that, so it won't remain so open-ended.
Below is a pix of the 2 different spark advancers I referred to: the one on the left is the K1-K2 advancer, until late 1972-early 1973 (K3 series bike arrived), when the one on the right (manufactured by TEC) appeared. This later one remained in production until the end of the 750K series. I don't know if it is the same as the "F" bikes' advancers.
The left spring on the K2 unit is a little deformed from my rebending of the end, after cutting off 1/2 turn of the coils. It still pulls the weights fully closed, though. The K2 advancer is smaller, lighter, and has lighter weights, and the spin diameter of the weights is smaller by 2mm. It has smaller diameter wire in the springs (.030"), and the diameter of the spring itself is 1mm smaller, as compared to the K3-later version. The K3-later one has .036" diameter wire in the springs.
Sharp viewers will note the difference in the points cam index markings: they are NOT interchangable, but the springs and weights are. In fact, they are 180 degrees opposite each other in the cams.
You can tune your advancers with these tips: using the K3 weights on the K2 advancer will net 18 degrees advance, while vice-versa makes about 36 degrees advance angle, presuming you don't bend the stops in or out. Using the K3 springs on the K2 advancer will slow down the spark advance, peaking at around 3300-3500 RPM instead of the stock 2500 RPM. Vice-versa: the full advance comes on about 2000 RPM.
Another thing that's not immediately apparent: the K2 in this pix has the ramps "stoned down" for gentler opening of the points, while the K3 unit actually has steeper-than-before opening ramps. The former was done for my RR hi-RPM excursions, and WILL increase points wear unless you use the Transistorized Ignition because of the slow opening: but it will also make for much more solid sparks, with no points bounce, to the end of a stock tach. It idles poorly if you do this without electronic assist, too. Honda steepened the ramps to reduce points wear and increase electrical life while lowering the effective redline, at the same time they were shortening the valve guides and installing seals on the intake guides to increase the oil change interval (sales team at work, here...). The drag on the valves made the Warranty engineers worry about pistons and valves, so the steeper opening ramps, along with lighter points springs on Honda's own points, caused points bounce pretty reliably at about 9000 RPM. This became a cheap rev limiter for the conservative-minded engineers. We modified a LOT of things on the points systems to get past this artificial wall that was not there on the K0 bike.
