They are not flat slide.. But not a full round CV design either. Kind of an in-between.. CV deal. FCR's are a great carb, and very much could be used in this app If one wanted to do the custom work of mating a set, working out the linkages issues, fuel issues. I very much doubt it is worth it as far as performance. Throttle response with an FCR is instant, with no lag at all. I've used them on several bikes, including a GSXR and my current toy a DRZ450

 

The VTR's carbs are indeed flat-slides but of the CV (constant velocity) variety. In essence they have flat slides but have a secondary slide that helps to control airflow (reacts to intake vacuum) so as to overcome the stumbling problems typically encountered in carbureted fours-strokes (when you crack opne the throttle, vacuum drops and so does the aux slide, tehrery keeping intake velocity decently high, once the engine vacuum builds up again, teh slide open allowing the engien to ingest more air).

Also, the unusually large diameter of the carb venturi on the VTR's carbs results in relatively low velocity airflow which makes the carb/engine combo all the more sensitive to hesitation and bogging. (note: non-CV flatslides work very well on two-strokes however). I don't think that replacing the VTR's carbs with non-CV units would be desirable unless you want to deal with all of the bogging issues (you would need a very smooth throttle hand to say the least). Accelerator pumps do help to overcome this bogging to some extent, however (squirt raw fuel into air stream to compensate for loss of vacuum and hence decrease in fuel draw from jets) .

Not sure if this is possible but I know that in some instances (snowmobile carbs) people do machine the venturis to increase the diameter and generate more airflow (perhaps an oval or offset bore would work best as it wouldn't alter the low-end too much but still generate more airflow on top-end)

cheers
Mikstr