Engine occasionally stops shortly after starting

A good while back, sometime earlier this summer, I experienced my bike stalling about 3 times in a row, shortly after starting it. The first time it stopped almost immediately after I let go of the starter. The second and third time I could easily tell it was idling way to low, so I tried giving it a little gas, but that made it stop again. On the fourth try, I  revved it a little bit, and it seemed to idle fine after that point. Today it did the same thing. First after filling gas, then 15 minutes later after I had been in a nearby store.

I'm not sure about the first time, but today at least, the engine was not properly warm. I had only ridden for 5-7 minutes to get gas, then another 2 minutes to said store. I checked the air filter a few weeks ago, and it looked fine. I didn't take it out to thoroughly inspect it, though. No mods, except a full LeoVince X3 exhaust system, which I'm pretty sure needs repacking, and a 47T rear sprocket.

I've never checked the valves or anything like that on it. It's gone about 18.000 km, or about 11.200 miles. The bike had about 7500 km on it when I bought it, and all the scheduled maintenance had been performed by a workshop. All I've done, and needed to do, as far as I know, is change oil, filter, and that sort of thing.

One more thing; I'm not sure, but it sort of feels like the bike might be getting a little weaker. I don't know if that's true, or if it's just my imagination. When I first put the 47T rear sprocket on, it felt like the front wheel would come up no problem, but now it won't come up until I hit the sweet spot of, what, 6000-7000rpm? It's entirely possible this is how it always was, though; that it's just in my head. Or maybe I just need to repack the silencer.

So, any idea what might be causing this?

EDIT: Looks like the pipe is chambered. After a bit of googling it seems this thing doesn't need repacking. I would've had to bore out the rivets, which I thought was weird.

I've had a spark plug go bad on me and cause the same stalling out issue. Became more frequent as the plug was going bad & eventually wouldn't make a spark. Another thing could be some bad gas from a particular station? Could try a different brand or station and see.

air, fuel, spark

Easiest things first.

Check air box and air filter. Clean and clear of obstructions?

Gas. Fresh and of high quality and proper octane?

Battery. New, and/or fully charged? Connections tight and clean?

Harder part is spark. A new plug might be in order.

There's been more than one story of a cracked ceramic plug causing problems.

Please be sure to update thread if you discover the problem.