When my car is warm the throttle does not return to idle. Sometimes with a few blips the revs do drop.
If I push down on the linkage with my hand the revs do return to a good idle. I don't have to push very hard.

Would a new pair of return springs help or do I really need to rebuild the carbs with a new shaft?

 

The steel shaft sits in bushes which are made of brass and any wear would have the effect of allowing air in from the side of the carb rather than the mouth. More air would raise the idle but pushing the arm down would be shutting the butterfly rather than sealing the worn shaft in its bushing. There might be some wear in the bushes but if it is bad you should be able to feel the shaft move up and down in the brass bush. The bushes will wear long before the steel shaft.
First I would reset the butterflies by releasing the clamps on the shafts, lubricating the bushes with a bit of 3 in 1 oil, unscrew the butterfly stop screws (idle screws) then let the butterflies snap shut on the springs. Once they are both seated tighten up the shaft clamps and adjust the idle screws to the correct settings.
You could also have some wear in the butterflies as well if they have not been set correctly on the idle screw in the past. There are two brass screws that hold the butterfly to the shaft and by releasing these screws a fraction and then snapping the butterfly shut they might reseat into a better position before retightening the screws. This has to be done with the carbs off the car as access to these brass screws is not really possible on the car. The brass screws have a split end so the screw cannot fall out and enter the engine so make sure these split ends are tweaked open.

 

If it were mine, I'd disconnect the pedal linkage to test what it's like by just pushing the carb levers. It may just be the links between pedal and carbs hanging up somewhere, round the carpets or whatever, so I'd isolate what area the drag is actually happening in. Sometimes they just need the clamps loosening and re=tightening to square things up again.

I mess around with my S Type carb links and springs as little as possible because it usually involves breaking my arm in about two places to get to them.

Ray

 

As Ray has said check the pedal linkage and give this a good lubrication as well especially the hinge in the bottom of the of the accelerator pedal which can become stiff. There is also a bush that goes through the bulk head which supports the throttle linkage and this can deteriorate over time.

 

Be sure that both throttle return springs are in place. There's a big one that goes down to the oil filter block and another to a bracket.

 

I remember the first time I took the throttle linkage off my S Type, when overhauling the cylinder head about four years ago, I had the Devil of a job sorting out how it all went back together a couple of weeks later.

When pulling it off, it seemed very straightforward and obvious so I didn't make any notes or take a photo. I could find no helpful pictures or diagrams in any of my half dozen manuals and instruction books so had to figure it out by trial and error. We usually have to learn the hard way!

Ray

 

As others have said: Check and oil linkage first, then check carb return springs. IF springs seems questionable, replace them. You may not have a major issue at all. Removing the carbs would be the LAST thing I would do, only if all other solutions fail.
Good Luck !
Schmitty

 

Thanks to everyone. As Schmitty remarked I do
want to avoid removing the carbs if possible. I will give everything a good lube and order two new springs.