I posted needing help, I thought I had a mix problem, may still have but:-
I thought the tell-tale was Ok when onboard, until I fired it up in a dustbin last week.
The engine runs in a well on the boat but I could hardly see the telltale output and thought it was ok because it was the last time I took it home, a few moths earlier.
So now, with a trickle, do I change the impeller or is the water passage blocked ?
The engine has had little use, 20-30hrs since new, but never been flushed.
The pee pipe is only about 1mm so I feared the thing was blocked.
After a lot of research on the net and expensive salt remover ideas I came up with this one.
A tub of hot (not boiling) water, I ran it in this for a few minutes, at higher revs the telltale definately increased, so I thought the impeller must still be ok.
Didn't run it for more than 3-5 minutes as I think my bucket wasn't deep enough to run properly and didn't want it to overhaet with hot water. I stopped it, put it back in the dustbin of cold water, now the tell-tale is as new. Have ran it from cold several times today in cold water, and it's as fast on startup as when hot.
Incidentally, I think there is no thermostat on Mercury Mariner 4hp sail motor 2007, the telltale is as fast cold as warm, I now know.
I was too coward to strip it down and replace the impeller, some articles said do it in forward gear, some said in reverse, and I didn't want to ruin a £700 engine.
I will bring it home and flush every time now, and have learnt my lesson.
Thank you all for your thoughts.
1 more thought, I did wonder whether to put some building or sharp sand in the water, to really give it a good clean, I guess sand in the sea does the same job.
Any thoughts on that would be interesting.
I hope this info helps someone out in future.
Sorry if I wasted anyone's time, but I did discover this so I thought I would share it.