Makita gas trimmer RBC221. engine runs fine but trimmer head does not turn.

Remove the angle drive gearbox near the string head and spin it to see if everything is moving properly. You may have stripped the gears out of it.

if it has a curved shaft check the cable they break went over loaded on both models check the clutch

Are the drive shafts engaged properly? you need to remove all the plastic engine covers and handle to access the clutch. I can send you the IPL if you need it.

Possible air leak in the fuel line, or the fuel tank vent isnt working and is creating a vaccum

I would like to suggest that you follow the warranty instructions for this machine and go about getting another unit. I'm sure if it is a Makita, that there is a valid warranty for it. The clutch is centrifugal and if it is not working, much disassembly is required.

The arbor is twisted into

Have an assistant start and hold the trimmer. At idle, there should still be a little drag on the clutches to make the shaft spin on that split shaft trimmer with the accessory head removed. No need to rev it or do anything fancy, but teamwork will make it safer.

Now forget about the engine and clutches - does the accessory head spin freely when turned by hand (removed from engine still!) and with a clear rotation of the shaft end visible inside the connector socket? Is the shaft end square with sharply defines angles, or rounded off? Does swapping in a different accessory function normally and as expected?

Sometimes the nub end of the shaft wears. Worse, the socket on the engine side has eaten itself to pieces. Worse, worse yet, clutches in the engine side that engage the accessory shaft are worn and there is simply no transmission of power. Good luck, hopefully this info will set your boots in the right direction!

usually it would be about 50:1 in todays engines, but if you are unsure mix 40:1 wouldn't hurt it , may run a little rich,

Cut 1/8" off outer (aluminum) shaft. Lube inner shaft with STP or heavy gear lube this will re-engage worn square drive .Replacing inner cable is un-necessary this is normal wear. Sears Master Tech BB

pull the shat off the engine and check the inner shaft it sounds like its stripped or the theengine side is cracked hope this helps