I was working on some SV650 CV carbs and Gpz550 CV carbs the other day I just had a thought.
Both of the above carbs have an enricher circuit instead of an actual choke. When you pull the choke, a valve opens to allow for more fuel to flow, but also, another valve opens to disable the slide from opening up under light throttle. On both of the above carbs, I found the problem was the choke were always active, even though they appeared to be off.
In the case of the SV, the choke cable was pinched in between the frame and a carburetor bracket. This caused the choke to stay on all the time. The main symptom was the slide would not open due to the vacuum valve being in bypass mode, preventing vacuum from lifting the slide. This yielded a mixture so rich it wouldn't even burn. The SV was only running on one cylinder. Freeing the choke cable fixed it.
In the case of the Gpz, the detent ball holding the choke rod in the off position was lost, so with vibration the choke would come on ever so slightly with the mixture ending up very rich and possibly hampering the slide operation.
If you still have the defective 750 carbs, I'd look to see if there is a positive feel to the choke rod turning off.
