Loosen the idler bolt and allow the idler to tension the belt. Rotate the crankshaft two turns and tighten the bolt. That will put that belt in correct tension.
If you look you will see a rubber plug in timing cover. You can tension the belt without removing anything but the plug in the future. Take a 12 mm socket and ratchet. Back off that bolt . Again rotate the crankshaft two turns and tighten the bolt. I did it all the time when I worked for a living.
Correct tension is 1/8" - 1/4"Deflection at 10 lbs "push". Have to feel it out but 10 lbs is not much. I just let it set auto with the tension spring.
Now you said you bought a new nivel carb ? can you upload a picture of it at a close up view.