Does your stock throttle cable fit snug into that brass fitting in the top of the carb? I think the problem is with the cable length and maybe an air leak on top of the carb where the throttle cable goes in.
Try this, take off your stock throttle cable and measure the cable sheath and the actual inside cable. The sheath should be about 41" long and the inside cable about 43".
Now measure that elbow pipe from your old carb including the screw adjuster that screws into the elbow pipe. I think it equals about 2" in length
Subtract the length of the elbow pipe and adjuster from both the cable sheath length and the actual inner cable.
Stock sheath 41" - 2" (length of elbow pipe and adjuster) = 39" New cable sheath length.
Inside cable 43" - 2" = 41" New insdie cable length.
Measure the inside diameter of that fitting on top of the NEW carb.
Now take your stock cable with you and go to a local bicycle shop and have them make you a new throttle cable with the new shorter lengths. (39" and 41") Try to find a cable sheath with a wide enough diameter that will fit snug into the brass fitting on top of the new carb.
You will have the file the button on the end of the new cable down to match the button on the end of your stock cable so the new cable button will fit properly into the carb throttle slide. The button is easy to file down, so file a bit and then compare it to the stock cable button and also try it in the throttle slide to see if it will fit.
Once you have the throttle button filed down the proper size, then install this new cable and sheath from the carb top up the frame to the throttle handle. Leave the top off the carb with the cable slide and spring connected. (this is so you can visually test it out of the carb for tension when you get the throttle handle end of the cable installed)
Slide the plain end of the cable into the throttle slide that is under the throttle handle grip and snug up the cable sheath end to meet the metal of the throttle handle perch. Push the inner cable through the hole in the throttle slide and pull the excess through and leave it sticking out by the throttle grip. Screw in the little securing screw in the throttle slide to secure the cable
Now twist the throttle grip and observe the CARB throttle slide and spring to judge if it looks like you have the correct tension.
When you think you have it right, install the carb throttle slide into the carb and tighten the carb top down.
Push down on the choke lever and twist the throttle all the way to see if the choke lever pops up properly. (later You can switch the choke throttle lever from your old 12/12 carb onto the new carb if you want)
Now fire up the Grande and test the throttle with the engine running and the bike on the center stand. Let it warm up a bit and adjust your idle screw on the side of the carb. If the engine is idling too fast or turning too many rpms, loosen the securing screw up on the throttle grip end and pull a little cable back for slack and then retighten the securing screw. Once everything seems OK, then take it for a test drive.
You may have to adjust the tension on the carb throttle slide by loosening the screw up on the throttle grip slide and pulling a little more cable through one way or the other. Pull it through to increase the tension and rpms OR pull some cable back through to lessen the tension and reduce rpms.