The deal IS that old gas makes "crud" in the carb. I hope you drained the tank and carb already and put in fresh gas.
Yes, you can note the position of the idle mixture screw by gently turning it in all the way.....and then remove it and spray some carb cleaner into the hole....and then put the screw back. (Be careful when spraying into that little hole; spray-back can be BAD on the eyes.) You can also put some cleaner in the gas (about twice the recommended rate) and spray some down the throat while it is running.
Even after all that, you might still need to dis-assemble and manually clean the carb. The cleaner in the gas will take a couple of days (and a couple of warm up runs) to do its job.
You ARE using the choke, right ?? Not only is it needed to start but probably to keep running until it warms up.
What is an SP125 ?? Dirt bike ??
