If your model has a fuel pressure regulator with a rubber diaphragm, these can leak and allow excess fuel into the inrtake. many of these have a tube connected manifold vacuum to it, and any leaked fuel gets sucked into the engine. Check if your model has one of this type - if so, locate the vacuum pipe (usually a rubber tube ) - pull the end off the engine end, and see if its wet, or smells of fuel. If it is, it needs replacement. Does the exhaust smell of petrol or is sooty?