Throttle won't return

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Mar 21, 2010
I'm working on getting my '75 MKIII ready to run for the first time in over 10 years and the throttle doesn't return on it's own and was wondering what the possible problem might be. I just bought this from my friend about a month ago and he had the carbs rebuilt many years ago after he parked it. Anyone have idea what might be the problem?
There is a screw on the throttle which controls tension on a friction spring inside the throttle. Maybe it is screwed in. (This was a crude form of cruise control.) I do use it to help with wrist fatigue on a highway ride. (wimp)

Lube cables? Are there return springs in the carb slides?
The cable is probably binding inside the housing due to corrosion (if it is original cable).

Check routing to make sure it is running without kinking or any wacky bends.
Wouldn't hurt to strip the carbs and make sure the slides are going up and down OK and drip feed WD40 into the cables, grease the handlebar end and the throttle housing too.

If you have twin Amals, the carbs have been apart and you aren't sure which slide goes with which carb, that can cause your problem.

Amal slides are NOT interchangeable, even on a given bike. They are lapped into the carb body one by one, so the LH carb slide might not work in the RH carb or a slide from a different bike won't fit either carb. They will drop in OK, but the close tolerances will not be right.

I almost got my butt seriously mangled when I didn't understand this wrinkle. My ride to work was a company hack 650SS. We did a study to see whether a single, larger carb would give better low-end torque which would help if you were pulling a sidecar.

Why anyone would use something called a "Suoer Sports" to drag a small caravan aboutwith three people in it was something I had a hard time understanding. Anyway, after we finished the tests (which showed that you could get a noticeable torque increase at lower rpm at the expense of top end power), it was my job to return my commuter ride to stock. I had never considered that the carbs and sides were individually lapped, so I just dropped one slide into each carb and went my merry way.

For several weeks on my daily rural commute between Kenilworth and the Marston Road facotry, I never had a problem since it was a lot of open throttle riding (what a blast). One day, I went into Wolverhampton on a comapny errand, using the 650SS. It was real busy, and I was weaving though the traffic as we were allowed to do in England back then. I went for a gap in the traffic (apparently successfully) and pulled in behind a double-decker bus. To my surprise, when I backed off the throttle, there was no obvious response - I still had a lot of get up and go. Fortunately, the bike had magneto ingition with a kill button right by the throttle twist grip. I managed to kill it before I ran into the back of the bus.

Back at the factory, I took things apart and found specific "comparison" numbers on the slides which specified which carb each slide was for and I had crossed them over when I reinstalled them. I don't remember how the numbers worked. I think this is called a "learning curve" but it's so far back in memory, I can't remember the details, but I sure remeber almost running into that bus!

If you're having problems with the carbs not returning promptly to idle, this is someting to check. If you can be sure that both slides and both carbs belong to the bike, just try switching slides between the carbs. This is a very simple test and it beats getting your butt creamed by "unavoidable full throttle" and is a simple first step in troubleshooting.

On a 125, unintentional acceleration is manageable. On a 650 or bigger bike it can be a killer. I was lucky that I had quick reactions back then. 45 years on, I'd probaly get killed in a similar situation, which is one reason that, at 69, I'm not riding any more!
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