Now I've got a '40 with no return lines and no emissions, so I may be talking out of turn here. But when you shut it off, can you see fuel in the sight glass on the front of the bowl?
If it is there (halfway) when you shut it off and not there 2 days later, the carb may be siphoning. That usually means you're due a rebuild because one of those little passageways is clogged or a check ball is stuck, etc.
I don't think there is a way for it to siphon back down to the tank, but then again I don't have two lines running to the carb.
If you're not having any other running problems, I wouldn't worry about it.
OK, I've reached my maximum level of incompetency, how about a '60 guy chiming in or at least a later model '40????
Fuel is siphoning out of the bowl into the manifold from the nozzles and this is not good. The reason for this is that the air vent holes (there are 5-6 of them) in the top of the carb are plugged and this will make it also run rich. There are two fixes:
1) Sure fire. Rebuild the carb.
2) The "Oklahoma" rebuild. Hose out the top of the carb with spray type carb cleaner and let it sit for 5 minutes. Start the engine, rev it to 2,500 and hand choke it until it dies. (You put your gloved hand over the air intake to create a vacuum at the top of the carb to pull fuel out of the air vents and clean them out). Repeat as necessary.
[quote author=mwd link=board=1;threadid=7666;start=msg64468#msg64468 date=1069126690]
Yes, it starts out with fuel up halfway in the window.
This is a rebuilt carb, a few months ago.
I have tried leaving the gas cap off. I have tried pulling the hose off that goes to the carbon cannister.
Out of ideas!! Wondering about the fuel pump, it should provide a check valve function. Maybe I should pinch off a hose close to the fuel pump to see if that helps.
It isn't going to siphon back out of the inlet or the vapor return. If it isn't siphoning into the venturi, it may be boiling out if the cooling fan isn't working. Take the air cleaner off and see if it is dripping after you shut it off. It is a common rebuild mistake to forget to blow out the air holes in the top cover. I like to check whtm with a piece of nylon fishing line.