Does anyone's string trimmer seem slower & less powerful at full throttle in short grass but then rev up in taller grass and weeds?

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Is the string trimmer supposed to seem less powerful (slower, less kick, quieter) at full throttle in short grass and then rev up in taller grass and weeds?  Also, does anyone's string trimmer rev up and get more powerful when you bump the head?  The line is long enough, but bumping it always seems to make it run better for a while.  I called customer service and they said that shouldn't happen and that I should return it, but I love the trimmer and am reluctant to return it. 
It does seem to lose power intermittently, but more in short grass than tall grass even though the throttle is fully depressed.   
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Posted 2 years ago

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Lehigh Lawn Pros

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Hi Barbara,
 One reason that the trimmer will run better when you advance the line, is because the 
cutter on the guard then cuts the ends of the line sharp. Running the line with sharp ends
will always run and cut better than having frayed/split ends.

As to why it seems to lose power intermittently I'm not sure, but it may be self adjusting to the current load, which would probably increase runtime. My PowerHead with edger seems to do this.

As long as it's not causing a problem, and the power is there when you need it, it should be all good.

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Blue Angel, Champion

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Official Response
The trimmer speed should stay more or less constant as you work. If you really load it down it may over-rev a little when you first remove the load, but it should not vary so much that it sounds like it’s doing it intentionally. It’s hard to say with 100% certainty without being able to hear it.

I believe what they meant by “return it” was to exchange it for a new one at the store. ;-)
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Thank you for your comments.  To clarify a little, the engine revs up and stays revved up for few minutes after I advance the line, then it loses power until I bump it again, but less so if it's being used in heavy grass/weeds.  It's helpful to know that sometimes they self-adjust to the current load but also that they should stay more or less constant.  I will try it again.  I am a nearly 70-year-old woman and this trimmer seems wonderful for my large rural property (except the battery life has been about 20 minutes).  If I need to take it back I will definitely exchange it for another Ego because it is so easy to use - no vibration, light and powerful.  
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Oregon Mike, Champion

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I wonder if part of what you are noticing is the sound change due to the length of the string changing. I know on my 15" trimmer (split shaft, original model) as the string gets shorter the sound gets "deeper". When I bump more string out the pitch gets higher. Like going from a lower frequency to a higher frequency. Makes sense given the longer string. I don't really notice a speed change unless I really bog it down while edging or get a little too close to some inanimate object. I've gotten really good at judging string length by the sound so I know when to bump some more out.
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szwoopp, Champion

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MIke you beat me to the post.  The string length and grass height /thickness will change the sound dramatically.  This could easily be mistaken for the motor rpm.
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That's good to know.  Thank you!
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Anthony Weaver

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I noticed the power change. In my experience the sound change was due to the string length. Power slows as the string get shorter. It's like the machine is asking you to bump and get more string. It works much better and with way more power when the string is long.
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Thank you!  That really helps!