FYI, Mechanic in a Bottle completely cured my surging/idle-hunting/sputtering mower
I've used Mechanic in a Bottle at the beginning of mowing season in the past and had great results, but had started using less and hadn't gotten a new bottle in a while, so I only had an ounce or so left. I used that during a warm spell a month or more ago, but since then the mower started taking many pulls to start and was also surging/idle-hunting the entire time it was running (i.e. every few seconds the engine would rev up and down, up and down, over and over).
I was considering taking it in for its periodic $100+ tune up but then thought I'd give MIAB a more solid shot first. Bought a new bottle, poured two ounces into the fairly empty gas tank, pressed the primer bulb about 8 times to get the fluid into the carb (as per the instructions) and let it sit overnight. Primed it another 8 times the next day and let it sit for 20 minutes, then went out and pulled the cord, and...it started instantly. And ran perfectly. Woohoo! I'll definitely be going back to full yearly treatments now.
I don't own stock in whoever makes the stuff, by the way — I'm just a lazy-but-now-enthusiastic-again admirer sharing my reconversion experience for anyone else who might be dealing with a mower that's sullen or unhappy, especially coming out of the winter months.