The engine runs fine without it. What it is supposed to do is suck back in the droplets of fuel that come out of the carb during a certain point in the engines timing. This is supposed to keep the plane cleaner and help with fuel economy. Does it work? Some swear by it. Others say it's BS. For me I can't tell any difference in Saito 72s with and without it.  And , if it is such a great aid for the Saito engine, how come some come with it and others don't? At one time the only way to get the stack on a 72 was to buy the GN version. This is the one with the black crankcase and gold rockers. Or you Had to buy one from Horizon and add it. 
    On the 67" Fusion like mine. I needed some clearance in that area anyway. So why not leave it on? The Tetherite thrust plate I got from OMP tilted the engine away a little. But the back half of the carb was still going to be really close to the fuse.

David