Is it a 1200 GX with round top carbs, or the later type with square top carbs? The later types have more plumbing & more places to leak air plus a couple of air bypass adjustments, If you have the latter type then try to get a 180B SSS workshop manual as the twin carb setup is pretty much the same & the adjustments will be right there in print. If everything is OK then procede to plan "B" below

If you have the early stuff like mine, then just pop the hose off the valve & put your thumb over it with the engine running. If the idle returns to normal, you have your answer. This valve is the same as the stock 1200 one, & the last time i bought one, they were not expensive.

Removing one permanantly from a perfectly good street engine is what loosers do.

They have NO negative impact on performance, & anyone who has the faintest idea how they work would know that. They also help keep your engine clean, both inside & out.

I somehow suspect that you have a gasket [or other] leak into the manifold.
Good luck
Chris