Valve float is a problem occurring in four stroke internal combustion engines, where the valve springs are not sufficiently strong to close the valves in between the inlet and exhaust strokes.

This is often because the engine in question is running so fast that the valves are being 'told' to open by the camshaft so often that there is not enough time for them to close properly before they are pushed open again.

In severe cases, in engines with interference type heads, this can lead to a piston hitting a valve, usually resulting in the engine stopping very suddenly and terminally.

Valve float problems can be remedied by fitting the engine with stiffer valve springs, or lighter valves, thus allowing the engine to rev higher safely, although in the case of using tougher springs the trade-off is that the engine will have to use more of the power generated to open the valves in the first place. Stiffer valve springs are usually only a good idea in engines that have had other performance enhancing modifications installed first, as fitment of over-stiff valve springs can counteract the benefits of having a higher revving engine and actually sap more power than they add.

From a design perspective valve float can be remedied by doing away with springs altogether, and using a desmodromic valve actuation system a la Ducati. Desmo systems actively open and close valves, keeping the timing of opening and closing perfect. Another alternative suggested by drinkypoo is the use of rotary valves.