This particular line goes between tank and pump inlet for my R5C 

it has a plastic cylindrical doohickey, can someone please tell me what this doohickey is for?   It cost $ to add so it must be for something.  

come to think of it I think some of my DTs had something like this too.  

thanks 

Umm, just a guess, goes against a bracket or through a hole to distance it & stop possibility of sagging & kinking. Love R5's & miss mine--was 40 years ago though.

Just replaced the oil line on my RD 400F . It has the same lump .
On my 400 there is a spring coil that slips over the outside of the oil line between the oil tank and the lump .
Presumably to prevent kinking . Check a parts list to see if your bike should have the spring too .