Ryobi riding mower won't start

I have a problem with a trimmer that I bought 2 years ago. The trimmer starts very easy and idles smoothly. I start to use it and there is no problem; however, the more that I trim, the more that the engine seems to bog down. At the end of 10-15 minutes the head goes so slow that it is worthless to try to trim. I do not have the trimmer running at full thottle the whole time. (Example, I pull the throttle, trim around a post, release the throttle, walk, pull the throttle, trim around a stump, release the throttle, etc.) The engine idles at a normal speed when the thottle is released, it is just when the throttle is pulled and the trimmer head starts to turn does the engine seems to labor but it happens gradually.

I have a 20 HP Briggs and stratton on my yard machine that won't turn over. The motor (model 461707, type 0143E1) would move maybe a quarter or half turn then freeze. I can turn the motor by hand and it will turn over freely if the spark plugs are out. It is as if the starter motor doesn't have enough power to get it going. I was about to try cleaning the carb and using a rebuild kit but a friend had me check the coil first. We took it off and sanded some rust off and tested the fusable link. After I put it back on and reinstalled a freshly charged battery I got nothing! Now the starter motor won't even lift to engage the flywheel. I've tested the starter solinoid with a light tester (I hear it click when I turn the key), and also the fuse. I'm really new to engine repair. I read about testing valve clearance? but it sounded fairly complicated and didn't make much sense to me. Is it possible I just have the space wrong on the coil? How do I make sure the gap is correctly set? Thanks for any help anyone can provide!