So this mower is only 5 years younger than I am... I was having problems with it last weekend when I was trying to get it out. Charged the battery, checked the carb and checked the starter. Everything looked fine. I bypassed the seat safety cause I thought that was the culprit. Put everything back together and it started and purred like a kitten. It was to late to start mowing so I put her up. Yesterday I was going to mow so I started it up and went to engage the blades and it died. I think there is a safety switch of some sort at the end of the engage/disengage lever under the dash. That is a whole other problem cause I can't figure out how to take the dash off... please advise.
Clogged carberator. Basically the main jet is clogged. Engine runs fine until you put a load on it.
Remove carburetor. Carefully disassemble it (save the gaskets). Using a can of carb cleaner, blow out all the passages especially the main one (usually a tube sticking in the throat of the carb - small bend in the cleaning straw of carb cleaner is usually good enough to reach it). Reassemble etc.
I had one there was a piece of green corrosion in the main jet. It was hard like a piece of sand but it had the patena of oxidized copper (copper green). Not sure what it was but it was on an engine that was run dry and stored over the winter.
In the future, use fuel stabilizer in ALL your small engines. you never know when you are going to pack it for the last time unless you consciously winterize the motors.
If you find serious rust behind the mixture screw, consider buying a new carb. The Chinese ones on Amazon seem to work OK - make sure you buy one with correct levers and attachments.