my yardman mower only runs for 5 seconds?

sounds like your gas filter is plugged, must not be gettng any gas

Change the following things: Air filter, do not attempt to clean it (which you can) get a new one - cheap.

fuel filter if you can find one on it, again, get a new one - cheap.

Spark plug. Go to the lawnmowever store where you are going to buy the whole mess above, tell them what make and model you have, they will sell you the right spark plug, then all you need is a wrench for it.

All of that will take you about an hour or so to do, but if you took it in for repairs, it is about $40-50 bucks an hour and several weeks of waiting.

If it runs for 5 seconds normally means the ignition system is good. So you have to find why it is not getting fuel. Sometimes if the fuel filter is plugged it will let enough fuel through to start but then dies when no other fuel is available. If fuel filter is plugged be sure and drain tank as it is probably full of trash. If fuel filter is not problem you will have to find problem with carburetor. You may have to disassemble and clean (or have it done) as the screens and jets may be plugged.

Runs for 5 seconds also tells me the ignition system is ok....

Take the fuel line off at the carb... let it go real quickly and watch for a steady fast flow, if thats good.... then its the carb (requirements for a good carb cleaning is a rebuild kit, tear it apart, soak it in carb cleaner for a few hours and blow out all holes with compressed air)

It could be if you haven't used it for awhile your gas in the tank went bad. You should be able to use pliers to squeeze down on the clamp holding the gas line on and pull it off draining the bad gas. Put fresh in it and let it mix, it should be ok.

Check for fuel shut off valve or a cloged fuel line

need more info, could be gas, plugs, starter safety switch, carb, etc

fuel filter or air filter clog. change or clean them and try again.

Put gas in it or plug it in ?

needs an overhaul