I have a 31 cc ryobi weed eater start for about

I think a new carburetor kit and a good carburetor tune up is in order my friend. Sounds like it's starving for gas. If it bogs down after warm up when you throttle it, then yes it needs a carb tuning. The L screw is the low side screw and should be turned in lightly until it seats then back it out one and quarter turns (1 1/4). The H screw is your high speed screw. Also, turn it in lightly until it seats, then back it out approximately a 3/4 turn. With these settings the unit should start and may even stay running. Again, if it bogs when throttled, while running turn the L screw out a little more maybe only 1/16th of a turn and then try it again. If it does rev then you'll have to adjust the H screw in about a 1/16th of a turn. But make sure you only do this after the engine has warmed up.

Hope this works, good luck
Skidooer