What follows is an overview of what I learned while replacing my motorcycle's throttle cable. While I'm no motorcycle expert (yet!), I hope you can learn from my mistakes and perhaps benefit from my research, since the info on these old bikes can sometimes be hard to find.
Step 1: Find the Right Replacement Cable
I ultimately found a suitable replacement throttle cable on Amazon. But not before purchasing the wrong cable from my local repair shop! I knew my bike had a two-into-one throttle cable, but I wish I had known that it uses the tiny nubs (upper cable in image above) instead of the "more standard" larger ones pictured at the bottom of the image. Does your bike take the larger kind? Because now I have an extra...
Step 2: Tools You'll Need
This project didn't require any unexpected tools, just a screwdriver for the throttle handlebar pod, and vicegrips took the place of any teeny wrenches that could have participated. Lubricate your new cable before installing it. You may wish to wear gloves. If your fingers aren't super nimble, you may wish to employ a pair of pliers as well, for the springy parts.
Since you need to remove the gas tank, be prepared with extra fuel line/filters on hand, since if they're old they could be brittle and break when you remove them from the tank. You'll need a gas can or other container to collect the drips of fuel that remain in the line after you close the petcock.
Step 3: Remove Old Cable
After removing the gas tank, begin the cable removal process at the tops of the carburators. Unscrew the threaded ring and carefully pull the entire throttle slide assembly out of the carb.
The next part can be a bit tricky. Compress the spring up against the lid of the carb to create enough space for the cable's nub to be removed from its groove on the slide assembly. Be careful not to let the screw go flying!
Repeat for the other carb, then unthread the cable from the fork. Open up the handgrip pod and bring enough slack into the cable to remove the single nub from it's groove on the throttle. Unscrew the elbow bendy section from the handgrip pod, and set the old cable aside (Mine came in handy again after I thought I was done with it, so I don't endorse throwing the cable away just yet.)
Step 4: Install New Cable
Installing the new cable is just the reverse of taking out the old one, but don't forget to lubricate the new cable first. Begin at the handgrip pod. Thread the cable slack up through the pod and screw on the elbow bendy part of the cable. In hindsight I shouldn't have screwed it in so far, as it affects how much adjustment slack you have in a later step. It's easy to undo and redo however, and you better believe I took this thing out and put it back in at least five times before the day was over.
Next, catch the cable's end nub in the slot on the throttle, and screw the pod back together. Route the cable through the fork, along the frame, so that the two ends are near the tops of the carbs. Thread the end of the cable through the ring and lid, then carefully compress the spring against the lid until you have enough slack to insert the nub end into its slot on the throttle slide. You may wish to use pliers to hold onto the cable if you find it difficult to hold the spring compressed with just your fingers.
Step 5: A Newb Mistake
When putting the carbs back together, I made a critical error. See, the throttle slide assembly fits into the carb two ways, 180 degrees from one another. In one position, the slide rests high inside the tube, but it is possible to still close the lid. However this is the wrong position! The slide should rest deep inside the tube. The difference between both positions is illustrated in the above image.
What happened when I assembled the carbs wrong? Well, this incorrect throttle slide position was effectively all the way open. So when I tried to start the bike, it just revved like a bat outta hell. Scary! And it took a while to figure out what I was doing wrong! So if this helps one person not make the same mistake, it's worth the embarrassment of admitting near defeat.
Step 6: Adjust Free Play & Enjoy
Once you've reassembled the carburators, you can put the gas tank back on and reconnect the fuel lines. Be extra careful that you haven't pinched the throttle cable anywhere-- give it a tug in a few places to make sure it can move as it needs to. Turn the handlebars right and left to see that it's not pulling. A pinched throttle cable can make your throttle open up when you turn the handlebars, which naturally could send you out of control.
Rotate the tension adjuster to remove free play (slack in the cable allowing the grip to rotate some amount before engaging). Someone told me there should be 10-15 degrees of free play on my bike, but I have no hard evidence of that suggestion's validity, and in my research online, the modern throttles I've been seeing have zero free play. I adjusted mine so it felt like it did before I replaced it, since that seemed to be working just fine. Too much free play means you have to rotate your wrist too much to hit the gas. Too little free play will prevent your throttle from snapping shut as it should. After adjusting the tension and with the bike off, try twisting and releasing the throttle to test that it snaps back to the closed position at far right and left handlebar steering positions.
I ended up adjusting my cable pretty much to the end of its capability, which is a bummer for the next time I want to adjust it. When that time comes, I'll take it out one more time and unscrew it one or two revolutions from the handgrip pod.
Thanks for reading, and please feel free to leave any and all motorcycle advice in the comments!