Why won't my weed trimmer accelerate?

There are a few possible reasons your trimmer will not accelerate. First, make sure your fuel is newer. It should be no more than 30 days old. If it is not drain the fuel tank and replace with fresh fuel. Secondly, check to make sure the cutting head is not bound with grass. If it is, remove all debris around the cutting head. And the last possible cause of this problem is a dirty air filter. Clean or replace the air filter.

I just want to add that there are three or more other possible causes to this problem that i know about. One being, a bad or worn clutch. Secondly, a loose or broken driveshaft and also worn components (springs, etc...) associated with the driveshaft. And, the last possible cause could be a bad gearbox or one about to go out.