1. See if the link below will help you.
2. Post some good close pics of what you are describing.
3. Follow the 2nd line to the tank to see if it is a vent or possibly crankcase pressure.

Please share or show your findings.

http://www.youtube.com/watch?v=uCC1qO75zDk

Thanks for getting back but my system is not like either of the ones shown in the video. There is no primer bulb and only one pipe going to the carb. The other pipe from the tank seems to disappear down the side of the engine. It might have been attached to something down there but isn't now!!

if you find a parts diagram, it might help. Please post a link to the diagram if you find one. OR! Pics of your carb and eng with bar and starter covers off.

http://www.mcculloch.com/ddoc/MCCI/MCCI1997_AAaa/MCCI1997_AAaa_04-MAC940_.pdf

It looks just like a fuel tank vent line.

Thanks for the parts diagram but I am nor sure if it helps with my current problem. The problem remains where the other pipe from the fuel tank should go. It is definitely not to the carb with my saw. IT seemed to be coming from a gap in the casing that led to the lower side of the engine. There is a brass ferrel in the pipe and I have convinced myself that it is some sort of system that uses pressure from the crankcase to pressurize the fuel tank but I am not sure where I got that from! Fact is despite pressure washing the engine, I just cannot see where the pipe should go. Any suggestions would be gratefully appreciated.

With a little dismantling where the line appears to be going will probably show where it goes. Follow the line is my suggestion. In my attachment the line goes from the tank to the carb. A minor coverer removal and line movement easily shows that.

If only it was that simple :-) I have disamantled the saw as far as I can. In my case the pipe does not go back to the carb. There is only one pipe connection to the carb so no return. As I said it seemed to go down a gap between the engine and casing but was not connected to anything. The end was damaged though so it looked like it had been.

1. Can you show a pic or or pics of where the line is going between the case and casting?
2. Where can you add compressed air to the case that might reveal the opening with air exiting?
3. Do you have a small engine shop in your area? I feel they would give you some quick info.
4. It is hard to believe with enough removed the opening w/n be obvious.

Good Luck!

On saws without a primer bulb the second line out of the tank is the vent line and just dead ends. Steve

Like I said, #32 appears to be just a tank vent, which would be that part at the top of the line, yours might have fallen off, a screw pressed in that line would probably do the trick,
stihl uses 2 set screws on their older saw. the screws keep fuel from pouring out of the hose, but will let air bleed into the tank, otherwise after running a minute or two, the tank will build up a vacuum and starve the carb for fuel.

Here is a link that might be useful: mac ipl

1. Have you replaced the bad fuel line and inspected the inside of the carb for restrictions, varnish, etc?
2. Have you adjusted the carbs and have a good feel for the process and reactions leaning/adding fuel?
3. How old is your tune-up? Approx hrs used per season? Approx how old and history of problems?
4. Is there a primer bulb. Does it show to move fuel?
5. Do you store with treated fuel or run bone to prevent varnish?
6. Have you tried 2 tbs of carb cleaner in your fuel?
7. Have you inspected the exhaust port, muffler flame guard X blockage? Feeling good compression?
8. What have I missed or you have tried? LOL