Join Date: Jul 2000
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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.