To change the clutch cable on you '79 Sporty, you'll have to take the outer primary cover off. To do this, you'll have to drain the oil from the primary, take the left side foot peg off, and loosen the primary chain adjuster on the bottom side of the primary. Loosen the locknut and use an allen wrench or hex key to screw the adjuster bolt downward.
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Then, remove the large "plug" towards the rear of the cover. Inside, you'll see something that looks like a nut but with no threads in it held in by a small spring. This the the lock nut, remove it. Below it is what looks like another nut with a slotted adjuster bolt sticking out. Remover the bolts from around the primary cover and break it loose from the engine. While turning the adjuster screw at the rear of the cover inward, remove the cover.
The cable connects to the cluch release mechanism. By turning the "link" downwards, you can disconnect it from the mechanism. Notice which way the "link" in attached to the release mechanism. Take the "link" off the end of the clutch cable.
Break the lock nut loose at the threaded cable adjuster on the outside of the case and screw the adjuster all the way out of the primary cover. Disconnect the cable at the handlebar lever and your cable is out.
Reinstall the new cable in the reverse order. When replacing the outer primary cover, make sure you use a new gasket on the cover as well as on the foot peg boss. The "foot peg boss gasket" is very important. You'll see an aluminum boss with a threaded rod sticking out of it that holds your left foot peg on. There is a round gasket that goes over this threaded rod and seals against the boss. If you leave this off, your bike will leak oil around this threaded rod when your foot peg goes on. Make sure you specifically ask for the "foot peg boss gasket" when you buy the new primary gasket. It's just a round gasket a bit larger than a quarter.
As you put the primary cover back on, there is a spring on the primary chain adjuster. Part of the spring is in the primary cover and the other part is in the engine. You'll see how it goes in. There is a large stud in that area that the outer primary cover slides up on. Adjust the tension on the primary chain by removing the "plug" at the top of the primary cover. You want 3/4" to 7/8" up and down play in the chain with the engine cold.
On the adjuster, as you're installing the outer cover, turn the ajuster screw back into the clutch release mechanism by turning the screw backwards. Once you have the outer primary cover completely installed, you must adjust the release mechanism. Adjust the screw inwards until you feel a resistance, then back the screw off about a 1/4 to 1/2 turn, insert the "lock nut" and spring, and install the "plug". Then, using the cable adjuster on the outside of the cover, adjust the cable. Add oil.