Step 6: Install carbonator,the heart
Next, you need to run the syrup feed lines to the fountain. Be aware that a few flavors, like root beer, will leave any aftertaste in the tubes so you can't ever use them for another beverage; You'd have to replace the tubing for the syrup feed. This tubing is typically 1/4" ID reinforced hose, as the pumped syrup is at a decent pressure.
Last step is to run the syrup source lines. This is typically slightly larger 3/8" tube. Put the correct bag-in-box style connector for that flavor on one end, and connect the other to the syrup pump. This is a very low pressure connection, and can use standard non-reinforced tubing. I would recommend, however, that you use something slightly stout (not the really thin vinyl stuff), because the suction could collapse the hose.
See this picture:
As you can see, the CO2 gas input is marked with an arrow. You can't see it very well, but there is a blue spout that is the CO2 exit. If you want to vent the CO2 outside your house for some reason, you would connect all the blue vent ports together and run that line to the outside.
The larger product "in" hose is shown top, and the slightly smaller output hose is also indicated. You can also see the black tabs that can be slid out to remove the fitting completely. Watch out for the rubber O-rings! Without them, you won't get a good seal.
Here, you can see that I have hooked up the six syrup pumps to the back of the unit. Notice that one in the middle isn't hooked up - now is a really good time to be VERY familiar with the diagram on the back of your unit:
If your unit doesn't have one, contact the manufacturer and get the manual, or check their website. You definitely want to have this information. Pay attention to the cold plate (rectangular block at the bottom of the diagram that everything passes through.) As you can see, starting from the left, we have syrup 1-3, an "open" connector, syrup 4-6, plain water x2, and soda water x2**. Be sure to follow this layout correctly. I suggest labeling all your hoses, syrup pumps, etc so you don't get confused and hook the wrong thing up somewhere.
** Plain water vs carbonated water: Most of the time, what they really mean is "shared lines vs direct lines". The "carbonated water" inputs both feed a "common" tube that connects to all the fountain heads except for the ones that have direct lines (plain water). The labels have nothing to do with whether or not you use carbonated vs non-carbonated drinks in each line. You can very well hook up carbonated water to all four and use only carbonated beverages. Or you could hook up carbonated water to the soda water inputs, and to one of the plain water inputs, yielding five soda water heads. Then put plain water on the second plain water input, and hook up pink lemonade or orange juice to the sixth head.
Notice in this enhanced diagram what is happening. The blue lines are the ones on the back of the unit marked "CO2 water" or "soda water". They are feeding fountain heads (valves) 1, 2, 3, and 6. However heads 4 & 5 are NOT connected to these inputs at all. They have direct lines to the rear connections, marked as "plain water 1" and "plain water 2".