Found this;
https://wolfgangint.com/blog/emulsion-tubes-n5
F11. This is the most common tube that comes in IDF carbs. It’s large in diameter so there’s not a lot of room for fuel in the well. It has a lot of air holes in the side so a lot of air can be mixed with the fuel after the well is dry. As you stay in the throttle the fuel ratio will continue to get leaner as the RPMS go up and load is increased. This tube is a great tube for a daily driver and most applications.
F2. This is one of my favorite tubes for racing. It usually doesn’t come in anything and you have to order them. It is a little bit smaller in size then the F11 tube, so again not a lot of fuel in the well. Just a hair more then the F11. It has fewer air holes in the side so it mixes less air as the RPMs and load rises. As far as air fuel ratio goes you will see it flat line or just slightly rise. This tube works great for race cars.
F16. This tube is slightly bigger than the F11, so it has less fuel then the F11 in the well. It also has fewer air holes then the F2 so it will mix even less air as the RPMs and load rises. When you look at the fuel ratio it will actually get richer and richer as the RPMs and load goes up. I have yet to get these to work on a VW engine, even in an all out drag car. In theory they sound like the way to go, but in reality they get so rich the car just bogs. Now I have run four 48 IDAs on a hemi V8 and they worked great. Go figure.
F7. This tube usually comes in an IDA carb. They are smaller in surface area than the other tubes I have mentioned. What that means is you get a lot of fuel at the hit. I find it’s too much fuel for most of the stuff I do. Some guys can get them to work, I can't. As far as the holes go in the side, they have about the same as an F11, so they get leaner as the RPMs and load rises.
I hope I explained this well enough. It is a complex little device in a carb that makes a world of difference in how your car runs. The biggest thing here I see all the time is floats being out of whack. The float level in these carbs is so important. If you’re not sure, check it.