I get a vortex of fumes when accelerating - and have had for years. Also I get petrol fumes on a full tank until about 20 miles then it goes away. All manifolds/pipes etc have been checked - fuel smell put down to breather pipe.
Check kingpins etc. for wear and possibly fit stiffer leaf springs & font springs - it sounds daft but it smooths the ride. Spanner Juggler is your expert on this for advice.
Do you mean the pipe that goes from top of tank back into the neck of the filler pipe, or is there another breather somewhere ?
