Under what condition does 'valve float' occur in a high-speed motorcycle engine?

1. When the valve springs are unable to close the valves quickly enough to follow the camshaft profile at high RPM
2. When oil pressure in the cylinder head is too high, causing hydraulic lifters to pump up
3. When fuel varnish builds up on the valve stems, causing the valves to stick open
4. When the exhaust valve remains open during the entire intake stroke