When float fishing for carp my strike rate is poor,
can anyone tell me if there is a correct time to strike.
The biggest thing with float fishing is to make sure you're ready. If you don't have your hand on the rod, or the rod in your hands then make sure you wait for the next dip of the float, and be ready as soon as you see it! Another point to make is to wait for the right bite... lots of this is based on judgement, but often small 'feeler' bites aren't proper takes. Wait for the float to go down and use your judgement as to whether to strike. If the float is going up and down and instantly coming back up then you'll probably miss it by striking. Wait for the float to disappear for a slightly longer period, then strike!
Another reason you may be missing bites is due to baiting the hook incorrectly. Make sure the point of the hook is exposed, and if necessary change hook size.