IMO not much point in changing things until you've found out the cause of the hiss. Might just be a split in one of the pipes under vacuum, possibly a servo hose but there are several to check. May mean you have to take them off one by one but a small split underneath is all it takes. With a leak the engine will think it's running weak and tell the ECU to add more fuel. Although things get hot pretty quick, a cloth in your hand can wrap round various joins an joints to show a change in the hiss or you can chase it with a length of hose. One end in the ear and the other moving around the top of the engine.
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