Well, I rode 8000 miles since spring, from Oregon to Minnesota and back, the bike worked perfectly the whole way.
After changing oil the starting problem began, would not start. Seemed like fuel was not getting into the carb.
I started it by rolling down the hill and popping the clutch and it started hard but warmed up and ran OK. But when cold, will not fire up.
My mechanic suggested installing a larger jet,
Forget the main jet. That only comes into play near wide open throttle. Not at starting with a closed throttle.
Check the battery and main cables by jumping to a car battery. If it starts investigate the battery and cable connections.
Check for fuel to the carb by opening the drain at the bottom of the carb and applying suction the vacuum nipple at the back of the petcock. Suck on it with a hose. If you get good flow out the drain, the fuel supply is good.
Check the carburetor function by squirting some starting fluid
in the air box near the filter. If it starts, clean the carburetor.
Since you say it runs OK once started, it doesn't seem likely to be a spark plug or wiring problem, but a nice clean plug never hurts.
Tight valves can make hard starting, but usually changing the oil doesn't cause the valves to tighten up. If you don't know the condition of your valves, it is a good idea to check them.