Everything ran fine in October and nothing starts in April, what actually happens to a small engine over winter?

There are two schools on fuel storage and they are both right, for different machines. The mistake is doing half of either.

Run it dry. Turn off the fuel tap, let it run until it stops, then drain the carburettor bowl from the drain screw if it has one. Nothing left to go off. Best for small equipment you will not touch for six months, and it is what I do with the trimmer and the mower.

Fill it and stabilise it. Fill the tank almost full with fresh fuel plus a stabiliser, then run the engine for a few minutes so the treated fuel is actually through the carburettor. A full tank has little air in it, so less moisture and less condensation on the walls. Better for anything with a metal tank that would rust, and for a generator you might need at short notice.

What does not work is the middle, and it is what most people do: leave a half tank of untreated fuel and shut the door. Maximum air, maximum time, no protection.

One thing worth knowing about running it dry: it does not empty the bowl completely on many carburettors, so a small amount is still sitting there. That is why the drain screw matters, and it is the difference between mostly working and reliably working.

Whichever you pick, do the whole thing.

It is the fuel, and almost everything else follows from it.

What happens chemically. Petrol is not stable. Over months the lighter fractions evaporate and the rest oxidises, leaving behind gums and varnish. It is also hygroscopic if it contains ethanol, so it draws moisture out of the air, and over a winter that water separates out and sits at the bottom of the tank and the carburettor bowl.

Why the carburettor is always the casualty. The passages in a small engine carb are tiny, the main jet is a hole you can barely see. It takes very little varnish to block or restrict it, and that is exactly where the residue ends up as the last of the fuel evaporates. That is your third year in a row, and it will be a fourth unless the storage changes.

Why it is worse in a dry shed than you would expect. Temperature swings in an unheated space push air in and out of the tank every day, and each cycle brings in new moisture. A sealed heated space would be kinder, though almost nobody has one.

The other two things that suffer: oil that has absorbed combustion by-products becomes acidic and sits on bearing surfaces all winter, and a battery left flat sulphates.

So the October job is fuel first, oil second.

The rest of the October list, which is short:

Change the oil before storage, not after. Used oil carries acidic combustion products, and leaving it sitting on the internals for six months is the avoidable part. Fresh oil going into storage is the correct order and most people do it backwards.

Take the spark plug out, put a small amount of oil in the cylinder, and pull it over slowly a few times to coat the bore. Then put the plug back. Two minutes, and it prevents surface rust in the cylinder.

Clean the deck and the air filter. Wet grass left on a mower deck holds moisture against metal all winter.

Battery on a maintainer if it has one, or off the machine and topped up occasionally.

And the buying decision that ends this permanently for the machines you use rarely: ethanol-free fuel if it is available where you are. It costs more and it stores far better, and for a generator that sits eleven months a year it is worth it on its own.

On your mower specifically, since it has been cleaned three times: check the fuel tap actually shuts off. A tap that weeps keeps refilling the bowl from a tank of ageing fuel all winter, which defeats running it dry and would explain why it is always that machine.