Le Mans III carb float chamber vents

Technical Big Block Tonti and Loop Frame models
1

After a rebuild my Le Mans started beautifully (larger jets as K&Ns were already on). One thing puzzles me though. Each carb has two brass nipples leading into the float chamber but only the inner port was connected to tubing that sat under the tank where the air filter box would have been. The outer ones seem to just sit there exposed to road dust and are presumably normally sucking air in?. At first when I turned the fuel taps on the floats did not close right away and fuel dripped out over the exhaust pipes. So, is this as intended?

The inner ports are connected to tubing to allow clean air in, but are the outer ones not blanked off inside to allow excess petrol to vent there rather than flood up through the carb or into the air intake? Any thoughts? I have looked at many pics and can’t see tubing on the outer vent ports on any bikes.

2

I will hazard a guess at answering my own question… the reason the vent is not tubed is to keep the height of the outflow lower than the air/fuel mixing chamber and simply flooding the engine while making the problem quickly visible. Perhaps the inner vent is simply there because Dell Orto could not be bothered to blank one off and so use the tubing to raise the height of the outflow and force any flow onto the visible side?

3

OK I’ll chip in :smiley: – all of these DellOrto’s seem to have two holes, one on each side. I suspect that all of yours originally had tubes connecting them to the air filter box, since removed. I’m guessing the two outer tubes went AWOL at the same time. Maybe the inner ones stayed behind 'cause was too much of a faff for the P.O. to get at them to remove them later. I’m just guessing.

The holes allow air into the float chamber so that the petrol can be sucked out by the venturi action. That’s all they do. Nothing to do with levels. Altho it’s true if needle valve is not seating fuel will overflow out of there. I agree it seems wrong to be open to the outside air thus allowing dirt and dust to get in, so much so that (years ago) I did a mod to my Spada to glue in little brass tubes, then I could attach thin-walled plastic tubing between them and the air filters.

4

Thanks Mike - that does make sense!