fuel/air mixture

Hello all, 
i have a GT550 1991 which i just got running, but now I am encountering a problem where sometimes it will rev super high when I'm driving, especially when pulling the clutch it seems like. I have cleaned the carbs once, and checked the intake rubbers for leaks and it does not appear to be any. I think my petcock isn't vacuuming properly, since its always flowing some fuel, and no difference if i suck on the vacuum line. The petcock have been apart and cleaned aswell. Could the petcock be a factor to my high rpms? Or am i in for a carb overhaul?
Thankful for suggestions
Best regards

With the fuel tap in the on position it should only supply fuel when vacuum is applied.
the only time it should be free flowing is if the fuel tap is in the prime position.
Unlikely to be your issue though. When was the throttle cable last lubed and checked for easy movement. No sharp corners or tight spots.
without the engine running fully open the throttle twist grip and let go. You should hear the carbs snap shut.

Better to start fixing what you know is not right, instead of speculating.   Petcock is probably vacuum operated; to open when in the 'ON' position,..meaning, it could be a source for vacuum leaks,...either it, or the tubing going to it.   Any vacuum leak is equivalent of opening throttle.

....If you know is 'always flowing some fuel', I'd either overhaul it, or easier yet; replace with a new one, and of course, check there is no vacuum leak in all of vacuum lines coming out the carb outlets,...  

I think Hugo and Warren have good points. The petcock is supposed to be supplying fuel to the carbs when running so if your petcock leaks fuel regardless, it shouldn't make much of a difference when driving. As Hugo stated though, you know its a problem so you might as well fix it when you can.

Definitely make sure that your throttle cable isnt getting hung up because that would be an issue. However, your issue is exactly the same as I had on my 440 when the carbs were dirty. Make sure the throttle is functioning properly and if it is, clean the carbs out.

If that still doesnt fix it, you could take a look at your petcock or recheck for air leaks. I like to use propane because it can easily enter through small cracks that starting fluid or most other fluids cant.