My lawn mower starts & revs up and down on a full tank,and when almost empty runs well!

Problem with carburetion. Check fuel filter, contaminated fuel, Air filter. Better fuel may be at top. When fuel runs out, runs better, possibly because it is running leaner(out of gas). Check float bowl if possible.

When I saw a red mark, I new you had less than 10 post, sorry 31.
It is picking up from the bottom like you say, good gas on top, when almost empty, runs good, carb can't get to top, until it uses up the bottom. Reread if you don't understand.
Why don't you provide answer mowerman2468.

Have you fixed yet? Don't know if mowerman2468's info helped. Didn't help me.