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If the fuel return pipe is obstructed it will cause too much pressure in the fuel rail and the regulator will be unable to deal with it. This will cause rich mixture. Especially at lower revs/throttle openings, as less pressure will be naturally relieved by the injectors due to the low fuel demand.
It could also be a fuel regulator fault? Although that would be rare as the pressure is pre-set by the poundage of the spring.. Not sure what would have to go wrong with it for the pressure to rise at low throttle settings.. I suppose it's possible to just stick?
It's hard to imagine why or how the return pipe could block up, but Hairyaxeman recently brought his S4 tank over with badly blocked vent and drain tubes.
We managed to get a piece of wire down one and partially down the other. He took it home and was successful in getting ever thicker guitar strings the whole way through the pipes.
What was surprising to find though, was that the return pipe was blocked solid. After some research and lots of help from the UKMOC elves, we discovered that there was nothing vulnerable on the inside end of the pipe, and he gave it large with various harsh rodding implements and got it clear.
Any help?
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