I had same issue discussed in this thread
http://www.diygokarts.com/vb/showthread.php?t=31838
So, I have 2 karts with 5hp Briggs that have the Pulsa-jet fixed-jet carb like in the pic in the thread (silver carb). Both karts had same issue. Both karts were bought used and had sit for some period of time (who knows how long).
I pulled the carbs, cleaned passages, even removed the Welch plugs and cleaned passages behind them with tiny wire, replaced diaprams, the whole deal. Still no success.
I got frustrated and went to Harbor Freight and got 2 Predator engines and put on them. DONE.
However, I took one of those Briggs and put on my own kart, but swapped the carb (and tank) from an older style Briggs 5hp (like the red carb pic in that thread). Runs like a raped ape.
Bobs4cycle forum is mostly all kart racers and the classifieds section is very active. You can run across the older style carbs for cheap if you keep eyes on the classifieds daily. Tanks too. You have to use the older style tank with the older style carbs. If you look, you can still find places that have the Raptor carbs and tank in stock (both are discontinued) but they ain't cheap, about $80 carb and $70 tank.
If you do get a used carb from a racer, you'll need to change the jet out to stock (36) or a 37-38 if you are gonna run gas with a header and high flow air filter. Not a big deal at all, less than $5.