Please browse thorugh the korean car forum regarding carnivals. I have a 2001 carnival with 50k km on the odo and it has no usok. I recently had the intake manifold cleaned for "gunk" accumulation. Dito pa lang solve yung problema. Carbon clean will only clean the fuel system but the gunk accumulation in the intake manifold will still be there.
Because it gunk accumulation thickens. it will sometimes block the intake valves which translate to lower torque. Have it serviced sa signet mas expert ang mga yan sa issue ng black smoke and loss of power. No need to change the injectors.