I purchased a reconditined Ryobi grass trimmer ZRRY34440 and I trying to understand how to used the unit. There is no spark plug on it . The manual mention a spark arrestor and a spark plug part number in another, So I'm trying to figure what is the right part for the unit before I use it.

Do you see any fuel leaking when you have it tilted? Grommet or gas gasket or hole in a fuel line might be pulling in air and stalling out. I've had 3 different line trimmers and that happened to all of them. Not a Ryobi tho. I thought they made drills lol. Jk

How to Install Lawn Trimmer Ignition Coil

Try not removing any parts unless you are confident of putting it back properly. Use a small pen knife to cut the trapped grass but be careful of your fingers.

Got a manual? They likely tell you there. If not:

Obtain and use the proper tools for removing the arrestor retention assembly/diverter and leaving the muffler in place right where it is at. Once the screen is in your hands? A blast of carb cleaner and a fine wire brush is all the attention it will need. Re-install, snug your retention screws/bolts and go play in the grass! :D

Remove your spark plug, take a rag and dry it up shoot a little WD-40 into it reinstall. Do not prime the unit or turn the choke on just pull it should start for you, make sure you are are not leaving the choke on, that is a common mistake.

http://odref.com/pdf/Ryobi/
If you have your model number, an owners manual should be here with correct mixture ratio for petro to 2 stroke oil.

To make sure that you have the right plug,you need to specify the make and model number of your trimmer.The first thing you need to do is check and make sure you have spark to the plug.
Set the trimmer down.Take out the spark plug.Put the plug into the plug boot and set it on top of the motor.Make sure the switch is on,and pull the recoil a couple of times.Look at the end of the plug to see if you have spark,and make sure it is blue.
If you have spark,you might have a problem with your carburetor settings.

I would say try adjusting the carb screws further out,or start by winding all the way in then out 1,5 turns.

my ryobi scorpion only has twofuel lines but two on primer can you help

Check the spark plug and sheck the gap on it then check the wire