Originally Posted by codprawn
Should the tell tale start straight away or do you have to wait for the thermostat to open or something?
On all the engines we've had the tell-tale has worked as soon as the engine has been started (allowing the usual few moments for the water to lift up the leg, of course). The thermostat doesn't open until the engine reaches temperature, so you can't assume that just because you have a tell-tale you will have cooling water for your engine ... if the thermostat fails it will still overheat even if you have a strong tell-tale jetting out.
I speak from experience
(even though ours was a bigger engine I imagine they work the same way)
So, sounds like impellor is the next thing to check - but a bit of a mystery what could have happened to it