B99 has the advantage of being nontoxic (well, at least 99% of it, and only until you use it for this purpose), but in my experience it doesn't clean the manifold as well as other solvents. And whatever you use must be properly disposed of as hazardous waste.

This is a very messy job. If you can find a local engine shop who'll soak it or steam clean it for $50-60, you're better off going that route. IMHO.