What jets are in it?. has it been cleaned? has the spacer mod been done ?- how?  what is your elevation above sea level?  did you plug the vacuum port for the petcock?   You need help -we need info!

Slide is sticking, diaphragm is leaking or someone totally f'ed up the jetting on the used carb you got.

Make sure you have the correct jets and that they are in the correct location.  It is common for people to swap the vacuum jets up top with the fuel jets on the bottom.  Don't go off doing any tuning until you get it running correctly.  Contrary to popular belief, most bikes will run decent enough with stock jetting... almost anywhere in the world.  I would start there or close too it.

My money is on the sticky slide/diaphragm leaking or not properly seated.  Mine would hardy go over 20 mph when it was sticking.

Where did you get the Petcock?

Don't take this the wrong way, but if you got it direct from Suzuki, you've got another vacuum petcock, which is what we've all endeavoured to avoid by using a Raptor manual petcock from Yamaha. Cheers!