The 1975 F0 bikes had the [first of] the poorer valve guides, which wear quickly. Typically, at 25k miles, they are finished. Not all F0 bikes have these, but many did, and all F1 and later engines as well.
That said, there's several other things that can make white smoke, from uneven combustion activity:
1. Leaking exhaust at the header pipe, on just one cylinder. You can replace the copper "donuts" under all that carbon to fix up this one.
2. Uneven ignition timing. Make sure you are right on the "F" marks for the 1-4 and 2-3 sets, at idle, and they both advance evenly.
3. Bad plug caps. They should be with 500 ohms of each other on the 1-4 coil or the 2-3 coil: on the K6 bike they were 7500 ohms when new and are worn out at 8500 ohms, while the F0 had 10000 ohm caps, worn out at 10800 ohms by Honda's spec. Replacing them should be done in pairs, and use the 5000 ohm (5k) caps from NGK for best results today.
4. One fouled spark plug. If #1 is smoking, look at #2, which is just ahead of it in the firing order.
5. Idle air screw set to the wrong number of turns. Set it at 1.0 turns, +/- 1/8 turn. I recommend 15/16 turn in most cases, to go along with the ethanol in our gas.
6. Too much octane in the gas can make it smoke. The F0/F1 and K5/6 ran on Regular in the old days, which is approximately midgrade with ethanol today.
