Excessive smoke due to a worn crankcase-seal that resulted the gearbox oil slipping into the engine.
Gear-lever seal leaked too, a cheap replacement.
Eventually a top-overhaul that I'd blame myself for, due to thrashing it every weekend for motocross.
My DT125 reads 'DT125E' on paper...and fondly refered as 'Yamaha Enduro' by the public. The 'E'signifies Yamaha's CDi ignition system - I guess it's the same bike as the DT125MX in UK? Back in the 80s, the monoshock was one off amongst scramblers, a pride!
Although a dual-purpose type, it can be seriously good for motocross-work. Mine was stripped left only the head&tail light. A YEIS induction from the street RX-S was bolted-on for better torque. Next was a bigger exhaust-system+carburettor -jetting. Knobby 'cross'tires. Eventually, it was 'upped'175cc by a simple used 175 block & piston. The primary & secondary sprockets were 13/52 if I remember. Wheelies in 3rd gear!
Around town it was fun, able to run with street RX's till 4th gear! People at the bus stands would cheer on when I rode up the pathway during traffic jams. On weekends, at the dirt track, some RMs & YZs could be trailed closely or even passed, depending on the rider!
For me, my youth means my unforgetable... Yamaha Enduro. I wasn't rich in money then, but truly a rich, exciting life! Mud, sweat & gears.... Period...