Don't know the exact set up, but a few thoughts.

If the cab operating ever travels a long way, but the valve does not, it suggests that there is either backlash in the operating rods,(or levers to spindles ) or they are flexing, and bending,rather than moving.

Alternatively, the lever ratios are wrong and not providing sufficient travel at the actual valve operating lever.

If there is a relay lever part way along the system, with unequal arms, is it connected the right way round, so that it magnifies the cab lever movement, rather than decreasing it?

Howard

Another thought: I don't know the locomotive but let's consider the geometric principles.

The ball-valve (if a rotating rather than lift type) needs rotate 90º for full movement, so 45º either side of a defined mid-point that itself gives full mid-points right along the mechanism. So at half port the handle is at mid-quadrant, irrespective of lever ratios.

If that symmetry is not there, the operating-links may become forced into trying to stretch or compress each other as they try to complete the ball arc.

I've an idea these larger BR Standard locos have a prominent reach-rod running along the boiler outside of the cladding, with a reversing-link part-way. In any such mechanism, irrespective of lever ratio or angle of arc, if the rods are parallel that link is ideally square to both rods at their mid-travel.

A similar principle would apply to the valve itself, which if a conventional ball-valve has the ball passage perpendicular to (closed) or in line with (open) the pipe. Usually the handle is parallel to the port, as an indicator; but if the valve is used in some other configuration it (or its replacement lever) may need be at 45º or even some other mid-travel angle, to it.

I'd suggest checking that the operating rod or rod end doesn't foul the valve lever or valve stem at extremes of travel. If someone made the lever too short (maybe in the interest of exact scale) it may foul the rod or rod end.

David,

I very much doubt you will need full opening on your ball valve. If you think about most regulators the steam port area exposed when open is not equal to steam pipe bore. Unless the bore of your valve is particularly small I think you will find you will be going like the clappers before it is half open. As long as you get an area equal or just larger than combined port area you should be fine.

Paul.