I'm puzzled with my line trimmer. When I put the saw blade on the trimmer, the engine will reach full speed and run really fast. But when I install the line head and extend the line to cut grass, it won't rev up at the same speed. I can't figure it out. The saw blade is heavier than the auto-feed line head.

As the song says, "the answer, my friend, is blowin' in the wind." The line on your trimmer catches a ton of wind as the line is spinning at a very high speed. Even though the blade on your trimmer is heavier, it catches less wind. Your trimmer revs up slower with the blade because of its weight. But it will rev higher after gaining speed due to the blade's flywheel effect and lack of wind resistance. And you might notice that just before you run out line, your trimmer speeds up. It makes sense; as the line grows shorter, there is less wind resistance. With reduced wind resistance, your trimmer engine runs faster.