My CB125S was running fine for many weeks this summer until I took a ride with someone on the back a few weeks ago. It wasn't the first time I'd done that, but this time, it seemed to be having trouble accelerating. The problem got worse over the course of the 20-minute ride and eventually the bike died just as I was getting home.
I let it rest overnight and tried to start it up the next morning. I usually engage the choke fully just to start the bike then flick it off to let it warm up a bit. I barely need the choke when it’s at its hottest outside but it usually helps to get the bike started on one kick. When I tried to start the bike that next morning, it wouldn't turn over with the choke on. I kicked it ten or fifteen times with the choke off and got it to start. It idled a bit but bogged and died as soon as I twisted the throttle.
I thought that it might be an air issue, like a clog in the intake, so I removed the intake hose (and noticed while I was doing this that the hose clamp holding the hose onto the carburetor had broken, but the hose seemed to be firmly attached). With the hose disconnected and the carb exposed to open air, I was able to get the bike to run noticeably better than before. I could give it throttle without it dying, at least. I still couldn't really get it to move forward without it stalling out.
Seemed like an obvious mixture issue so I tried adjusting the screw but didn't really see any change either way. I turned it up to a full turn in either direction, ⅛ twist at a time.
I pulled the plug and it was completely black and sooty, which I've read is the hallmark of a rich mixture.
The fuel is brand new and the battery is fully charged.
My question is twofold: How should I proceed in troubleshooting this issue? And what could have caused such a rapid deterioration (over the course of one 20-minute ride)?
