i need more information. Â
There are NO evenflo carseats that HARNESS to 100 pounds. Â Probably what you bought was a combination seat which converts from a harnessed seat, into a booster. Â Most of those style seats only HARNESS to 40 pounds, and then act as a booster seat - using the vehicle seat belt, until 100 pounds. Â
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Please give us the name of the seat, that will help, as well as the height and weight of your child. Â
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If it is an evenflo that is 2 years old, I am going to guess it is the Symphony? Â I believe the harness weight for the original was only 40 pounds. Â there is a new version that harnesses to 65, but I do not think it was out 2 years ago, its pretty new. Â
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Again, please give us the information, and we can help you.  Most 4 year olds aren't quite ready for a booster, so if she has outgrown her seat by height, yes, we would probably recommend  a higher weight harnessing seat as opposed to a straight booster at this point.  If she is still under the 40 lb weight limit for the seat, it should NOT be too tight...perhaps some belts have become twisted, or something else is happening...and if she is above the weight limit, it is imperative you NOT use the seat....the straps are NOT certified to hold anything over the rated weight limit for the seat. Â