Fatal???

Old gas is only an issue if its > than 1-2 yrs old w/o any additive.

The throttle adjustment plays a big role. Try moving it when pulling the string.

If this is a 10 yr old piece of crap, get a new one. Refurbs are

Well, my first question is; does gas run out the muffler when starts to bind? If so it maybe hydraulic locking (meaning combustion chamber full of gas and piston has no where to go). Cause the needle valve sticking and letting gas through the carb into the comb chamber and the piston is pushing it out the muffler

The only way to know for sure is to pull the spark plug removed. Now pull the rope. Should not bind. if it does it could be two things the winder binding and/or the rope catching or the crank, connecting rod, or piston hitting something. If I remember the 725r is a two stroke. If so it should have a crank case cover held on usually by 4 screws. Remove and inspect the bearings.

Usually the gas tank is dirty/gummed up and that dirt makes it way to the carb. and reeks havoic making it hard to start or no start. Some times you can just remove the gas tank clean it, change or clean the filter, disassemble the carb and clean all the passages.

If it over 5 years old then the gas (especially corn) has gummed up, plugged up and made the diaphram hard. This would require a rebuild kit costing about 8 bucks. Key is getting all the little passages cleaned out and installing the needle valve and diaphram correctly or it will never start.

I pulled the plug & it pulls smooth. I put in a new plug, cleaned the air filter, put in fresh hi octane gas. It's starting better. I have found the pull smooths out if I pull it slow & steady acouple of time. May try a rebuild in the fall.
Jerry