usually electric string trimmers takes thinner line .065 mayby .080 at the most. When I have this problem with bumphead-advance system it's usually the spring has colaped allowing the string to just spool out when ever you touch the gound with any pressure on the bump (spring release knob).

I usually just spring the spring out or stretch it to give it more pressure when you bump it. IMO that's the problem with the bump-advance stringers especially the light string models the spring colaspes alowing the reel to free spool if you have hard bump problem especially when the string gets knot on the spool locking it up and not advancing.

Thanks remoser. Could you explain a bit more what you mean by "spring the spring out or stretch it to give it more pressure" - I'm not clear what you mean by that, and if it might help, I'd like to try it.

Grasp each end in yer fingers and apply a steady pressure outwards, but don't go too far! A bit of pressure should suffice. Remember you must get the spring back in its place--can't have any hanging out! ;o)

Along with rustyJ comment all springs are made to certain tolerance. Meaning the height, diameter, and stiffness produced certain resistance. So measure your sping height. then stretch little at time to get the right resistance or spring pressure you desire to let the advance spool release one click at time when you lightly bump it. If you have to smack it on the ground usually the string will tie itself in knot and not release.

If the spring is colasped enough it will release with the slightest bumping. So to make this short depending on the model and design. the spring has to be certain height to alow the right tension on the bump advance. Too weak (or Short) line comes out to easy. Too strong (spring height longer that it's suspose to be makes string hard to advance (providing the string is not knotted up on the spool which happens from time to time). All of us at one time or another has bumped it extremely harder thinking I didn't bump it hard enough when all it was the string became knotted and locked up on the spool reventing string advancement.

The spring probably the second most important invention sence the wheel. First spring; rope tied to tree limb pulled tight to snare animal, long bow: spring device to shoot an arrow. spring lever cannons. Metal springs probably the most important device in modern era. Springs are every where: from watches, Tools, door levers, guns, lawn equipment are full of them, even simple ink pen has spring. Springs goes both directions! Some of us remember the old homemade screen doors that had the long spring attached to pull the screen door shut. Doors now have spring inside the hinge or inside cylinders to pull the door shut and air pressure to slow it down to not slam.

If the head is feeding out excess line- look for missing / worn tabs in the spool and drum. If the line is cutting off as it exits the drum- possibly the metal plate/ eye is worn or missing. The line tends to melt on the plastic directly. Depends on the version.