If that's the butt end of the
exhaust manifold, the blockage is a 'coke' formation caused by excessive unburned
fuel passing through the combustion chambers, sticking to the HOT side walls of the
exhaust manifold and slowly 'roasting' into coke.
Usual causes include bad injector timing, use of degraded
fuel, bad injectors, engine that is being 'lugged' (from an 'over pitched'
propeller) or any 'combo' of the above.