What kind of trimmer are you using the speed-feed on?
Is the threads on the output right hand or left?
It sounds like you have a right hand thread with the trimmer wanting to turn the head off when running. I may be all wrong here but I think most units are left handed threads and the machine turns in the direction that would make it imposable to run off. If anything it should get tighter as it runs.
Sounds to me you need the smaller 294 (3 3/4) size. Most of there are fleft hand thread.
Just my opinion and experience.