15.5hp Briggs OHV - '04 model - blowing grey smoke BAD

Common problem with that engine is blown head gasket. Will be blown between cylinder and push rod cavity. May also have a valve adjustment problem causing the hard starting, but you will have to readjust valve after changing head gasket anyway.

Pull the dip stick and sniff the oil - smell like gasoline ?

Just throwing these thoughts out to you:

White or blue smoke while running may be an indication of an excessively worn cylinder or rings or a clogged or inoperative breather (the breather assures that there is always negative pressure in the crankcase - if not, oil can get forced up into the cylinder).

was the mower left out in the weather, the air filter may be clogged, causing it to pull oil from the crankcase into the cylinder and then into the muffler,

Check the oil level. Sometimes too much oil in the mower can cause white smoke to blow out of the exhaust.
It is possible that gas contaminated your crankcase oil and you will need to change it to get it clean again

Is the spark plug is adjusted correctgly, or clogged with burnt oil or other crud (often from a clogged air filter), it can be impossible to start the engine. If you do get it started, smoke or other exhaust issues can occur


A carburetor will get clogged and dirty, especially if the air filter is clogged. Sometimes this problem can lead to one of the floats leaking or the needle seat going bad

Compression Test or leak Down Test might also be something to look at.