I replaced the fuel lines, bulb & fuel filter and added fresh mix and a new spark plug. After many pulls in various choke settings and bulb pumps it suddenly fired and after a few more pulls it came to life. While it was running I set the idle and tweaked the low screw and tested the trigger and few more times, let it run a bit, and then shut it off.

I re-attached the cover and put on the attachment. After many pulls it finally fired again, smoothly running the string trimmer until I shut it off.

Now it won't start at all. Not even a pop.

Ryobi will send for free a carburettor adjusting tool with instructions how to set the carb so it will run. Also check the fuel lines. These decay quickly and leak. New ones can be bought at garden machinery centres by the length. Fitting is not easy, cut the end obliquely to ease passing it through the tank wall.

Hi,
Sounds as if you need a gasket & diaphrgm kit in that carb.
Peace,
Clarke