At 10-15 kilometres per hour you're definitely going to want to be covering and feathering the clutch. As well, you probably don't want to be in 2nd gear: In 1st gear, 10kph is just 1550rpm; in 2nd, it's only 1000rpm. For perspective, idle speed is ~1200rpm for these engines.
As mentioned above, at those sorts of speed your riding should be a mix of feeding in some power via feathering the clutch and using some engine RPM, then pulling the clutch and coasting a bit.
Here's some food for thought: At 1000rpm, the engine is turning just 16.7 revolutions per second. A 4-stroke engine produces a power stroke every 2nd revolution of the crank so you're getting a power stroke from a given cylinder roughly 8.3 times per second. Because the crankpins in the twin are phased at 180-degrees instead of 360, you're actually getting one cylinder firing and then the other firing as the first's power stroke ends; it results in an uneven power delivery (180-degrees/540-degrees) that becomes very noticeable at very low RPM and low speed: A power stroke at time t=0 followed by one at time t=30mS and then nothing for the next 90mS; at 10kph and 1000rpm, you will have traveled 8.3cm over the road in the 30mS between the two power strokes and then 25cm -- almost a foot -- before the next power pulse. You're definitely going to feel this.
We tend to think of gasoline engines as almost seamless in their delivery, mostly because most of the time they're operating at high RPM, have high(er) cylinder counts and, in cars at least, have heavy flywheels/torque converters etc that smooth the power delivery. However, a small engine with a low cylinder count, a light flywheel, a rigid mounting to its frame, a manual clutch and a loosely-coupled final drive (chain slack) and an uneven firing pattern is going to make itself very obvious when tooling along at low speed, running at such a low RPM in gear with no clutch slippage.
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