what would make a lawnmower just start smoking & lose power, halfway though a yard?

Black smoke is generally a fuel issue. Carb is probably running too rich for some reason, needs re-adjusted or blocked airway somewhere. If it's more along the lines of a whitish/blue smoke, thats oil burning (broke/wore piston rings, grove in cylinder wall, etc...)! Don't listen to your #2 answer, there are NO rod bearings in a small engine!

Have you ever checked or changed the oil? Do it if you haven't. Possible water in gas, this will cause galloping or pulsing of the engine blowing gray/black smoke as rpms climb to normal. If you ran out of oil, fill it ,run it 5 to 10 minutes drain and refill. if it has a filter, gas or oil and air ,change them and hope you did no serious damage. probably not, these small engines can take a lot. If you can still turn it with the pull start or elec. starter it is probably O.K. and just needs a thorough cleaning. Good luck.

I agree - its definitly in the carb - sounds like its running rich or like suggested the oil breather could be puking oil in the intake.

Change the oil, check the carb , probably plug needs changing as well.

I have a 12HP Tecumseh motor with that issue - i couldnt figure out why the previous owner plugged the vent - once i opened it i found out why. Saturated the aircleaner element and blew out grey smoke. Thats why its sitting on my bench ( sides i dont like Tecumseh motors).

Kidd

I would suspect wet gasoline, or maybe you accidentally filled it up with oil/gasoline mixture for the weed eater/chain saw? Check the oil, replace the gasoline and run it again. I doubt that there is anything wrong mechanically. Modern small engines are almost bullet-proof. Of course, if you hear stuff clanking around inside the engine....it's toast.

Sounds like the air cleaner element needs cleaning or changing, depending on the type. Grey/black smoke usually indicates something is clogging the flow of air into the carburetor.

Sounds like a blown head gasket

Is a petrol (gas) mower that has run out of fuel?

Or a poorly electric one......did it blow a fuse? Did it get wet?

Leave it for a bit and try again when dry, if it's electric.

lack of oil

overheating

bearings worn out

any or all 3

crankcase breather went bad...take off your airfilter and if that little vent tube blows oil wen its running its bad...i work on them and im only 14 ... had one do that before

A clogged air filter can also cause this problem.