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TR2/3/3A Carb throttle return spring tension


Jedi Hopeful
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To fix the slow leak from the jet assembly on my 1959 TR-3A I decided to give the Superdry Jet Bearing and Seal kit from Moss a try. With the carbs pulled I noticed that the return spring on the throttle spindle on the rear carb was not under any tension. As a result, that spindle was free to float a bit. With the spindle connecting rod fixed in place both of the throttle discs were fully closed. That suggests to me that only the front return spring is under tension at low throttle settings with the rear spring coming into play at the higher throttle settings.

So, here's my question. Is the rear return spring supposed to be under tension with the throttle disc fully closed? If not, is there any guidance how to set the tension on these springs? I have not been able to find any discussion of this in the manuals or online.



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I set my springs under equal tension. I've got a couple SU books, and they do not mention staggering the springs either.

In addition, using the springs on the shafts are the preferred way to close the throttles. They put a rotating force on the shafts, so they do not tend to wear the shafts and bushings in the carbs. The springs that are connected to long arms provide more of a linear force, so in addition to closing the throttles, they place a side load on the shaft and bushings. This means the long spring on the firewall will wear the throttle shafts much more than the circular springs on the carbs. I would recommend putting the carb springs under a good bit of tension so you can minimize the linear springs.


Jedi Hopeful
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I would recommend putting the carb springs under a good bit of tension so you can minimize the linear springs.

That makes sense to me. I never thought about the different loads on the spindle and the resulting wear. There was very little if any play in the shaft when I checked it a few year ago, but perhaps it would be a good idea to check it again since I have the carbs torn down.

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