I have bought 3 replacement bump knobs for this trimmer, and every one comes flying off after about 5-10 seconds of use. I securely tighten them by hand but they keep flying off. Help!
What comes off, just the knob or the knob & the cover plate.
If it is just the knob that is pulling through the either the flange on the knob is too small or the hole in the cap is too big.
The the entire cap comes off then the tabs that hold it in have sprung.
Put it in some hot water for 15 minutes bend them out and hold them out till the cap / head cools down .
The small round knob that screws into the head (looks like a push mower gas cap). Has a bolt in the center that screws into the center of the head. Immediately unscrews when engine starts.
If I take the inner reel (the part that string wraps around) off and run trimmer with it off, the bump knob stays put. If I replace the inner reel, the bump knob unscrews itself and comes off. The bump knob screws in clockwise, the spool head spins counterclockwise when trimmer is running. With the inner reel removed there is no pressure from the spring against the bump knob which allows the bump knob to remain in place. With the inner reel in, the spring pushes the inner reel against the bump knob and keeps it from spinning with the rest of the assembly, thereby causing the bump knob to unscrew itself and fall off.
Is it possible the inner or outer spool ratchet stops are worn or broken and allowing the inner spool to rotate the wrong direction when you increase or decrease the throttle and unscrew the bump knob.
Here is a picture of the inner spools. the outer spool has something similar which slip past each other when you bump the knob. If those get worn or in some cases can get grass built up can allow the notches to slip past each other when you throttle up and down which will unscrew the knob.
Installed new string spool with string, and new bump knob. Bump knob flew off after about 10 seconds, just like before. Is the bolt in the bump knob supposed to turn freely inside the knob or remain fixed?
I don't think it is the string. The only other thing I could think is could be is the threads in the shaft where the bump knob screws into is worn, and loosing friction preventing the knob from properly tightening down. Something else to look at is make sure the internal thread cavity is clean. Maybe grass built up or something else causing a bottoming out effect.
When operating the trimmer, the string spool rotates in the opposite direction that the bump knob screws into the shaft. Looks like that will always unscrew the bump knob and cause it to fly off.
