I agree with the idle screw point . These were notorious for this. Spray around the hole mate to see what the revs do. The black stuff in you pipes is either carbon mate or the rubber is breaking down with age. Remember to be careful with the adjustment screw in the meter head when you are tuning and never ever rev with the allen key in place!! There is a trick when you have fuel pressure problems on these but try the simple things first with the can of spray.
Good luck