2 stroke oil goes in first....

It can but is not necessary because 1) oil will naturally diffuse from areas of higher concentration to areas of lower concentration



and


2) return line will mix the oil and fuel.


Putting it in 1st requires that you know how much fuel will be bought. I frequently buy about 10 gallons of fuel give or take and there has always been sufficient capacity to add 10 oz (1/3 of a quart) to the tank.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

I had a similar problem on my w124. I would fill up, drive home a few miles (ie, there should now be a bit less fuel in the tank, right?), and maybe 2 days later there would a fuel leak under the car that was like 16 inches x 16 inches, ie, pretty big. Couldn't figure out what was going on.

Had my mechanic look at it, and there's a bell shaped thing under the fuel tank. you might be able to see it without having to jack the car up if you get your head in there and look around. I forget its function, handles some fuel overflow or something.

End of story, unless I wanted to do some work under the car (I didn't), the solution was to not fill up to the very brim. Somehow what was happening is that if I fill up completely, some of the excess fuel goes into that overflow hose and down to the bell shaped thing. Now that the car is 28 years old, something is not working like it's supposed to, pressure builds up, and that fuel overage ends up on the ground.

Solution for me: As soon as I hear the fuel pump getting to where it'll shut off soon, I stop pumping right then. Problem solved in my situation.

Sounds like you are describing the Tank Vent pipe which on mine is in front of the drivers side rear Wheel. See pic.
Note this is someone else's pic. I think I have read they all don't have the same bell shaped end to them.

Also in the diagram 77 is a hose. I suppose it is possible for that hose to get old and kink and or collapse with age.

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84 300D, 82 Volvo 244Gl Diesel