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I have a 4 cycle Craftsman trimmer with a Walbro carburetor (751 16046 4 carburetor) the carburetor has 4 ports, one for the primer bulb to bring fuel into the carburetor and one for the fuel to exit back into the fuel tank. What are the other two for? Thank you...

Is trimmer Model #CMXGTAMDSS30? Lowes sells them. Go see a new one & check the fuel line routing. One port is is probably a breather of some sort.

Maybe one of these site will help you better understand how the carb works.

Here are some pics of the carburetor. Thanks...

It's a 4stroke. One will be impulse from the crankcase.