I’ve never driven (and probably have never even seen in person) an ICA motor, but I’m wary of statements to the effect of “direct drive equals locked-up clutch.”
Back in my bicycle racing days, I spent a few years’ worth of Friday nights on the velodrome in Houston (track bikes = single fixed gear, and Look, Ma! No Brakes!). All I can say is, riding a fixed gear is way, way more than just leaving your road bike in the same gear all the time and telling yourself not to coast. You just plain go faster on the fixed gear than you would using the same gear ratio on a road bike. Maybe it’s the reduced weight of the machine, or the lower rotating mass or reduced friction of the drivetrain, but it’s definitely a different sensation.
Switching back to karts, you may think your clutch is “locked up,” but the fact remains there’s a finite amount of friction holding the clutch shoes in place on the drum (sorry, folks, I only hablo Rotax). Shock loading can result in some slippage, even if it isn’t perceptible to the driver. In the drive direction, the clutch is self-energizing, but in the opposite direction it isn’t.
It may be that 99.9% of the time, yes, a locked-up clutch might as well be direct drive. But still, you have the extra rotating mass of the clutch assembly that you have to accelerate and decelerate, and you have a “mechanical fuse” (clutch slippage) the direct drive just doesn’t have if things get hairy.
That’s the Mechanical Engineer’s perspective, anyway… I, for one, would’ve found it much more difficult to get into karting as much as I have without the clutch and the starter. 🙂