craftsman chipper problem

[trailwart]

its got a b&s 5hp engine on it and there is some rust in the tank. i need to get rid of the rust, any treatment/preventative chemicals i can use. i seen a product called kleen kem is this the stuff to use. the engine is running hit and mis, sometimes it will run greatr in the run position and then other times it needs to be choked to run, i think it has rust in a jet or a peice here and there gets lodged and then passes through. do you think thats whats going on with having to run it with choke sometimes. thanks for your input