[Davehuge]

Changing the gearbox oil wouldn't cause the problems you've explained.

Try the simple solutions first, try a new spark plug.

Also could be caused by a small blockage in the carb, for example debris blocking the tiny orifice of the pilot jet.

However, I'm wondering if your ignition timing has slipped, that could cause the issues you're having.

If you read back through this thread you'll see Undertow's ignition timing slipped because the woodruff key had sheared, the woodruff key locates the flywheel on the crankshaft and together with the position of the stator plate, sets the ignition timing.

His bike was also a 2018 model. If the nut securing the flywheel was not tightened/torqued correctly it could allow the flywheel to rotate on the crank and shear the key.

Shouldn't jump to conclusions but there's a possibility there's a batch of bikes with this issue caused by a problem on the assembly line, for example if an assembly worker's torque wrench was faulty, not set correctly or he picked up the wrong torque wrench.

Let us know how you get on.

All the best, Dave.