1980 Ski Nautique overheating

Gary S wrote: Why the closed system? You running in salt or brackish water? The heat exchanger does need to serviced from time to time. Google marine heat exchanger flush.

Hi All,

First post - i'm from Australia. Have a 1980 Ski Nautique that hadn't been used for 9 months or so (kids, life getting in the way!), took it out and it got hot after about 6-8 minutes of runtime and actually stopped. I'm assuming the fuel was vapourising??

Anyway, got it home, and pulled the raw water impeller out and it was perished. So put a new one in and fired it up at home on the trailer. It ran fine, but when i revved it to 3500rpm or so for a minute or two, the temp got up to 190-200. Typically it would only previously get to this temp if you pulled a skiier flat out for a long period, then would drop back to 160-170 within a minute of backing off. On the trailer now, even when i got it back to idle (garden hose running through the inlet) it stayed around the 185 mark. So its running too hot, and i can imagine on the water under load it will get hot very quickly.

So: a few questions and open to any ideas on what to test/replace etc...

1. Could i have done something nasty to the engine when it got to 200 (only for a few minutes)
2. The raw water pump appears to be fine as water is being blown out the exhaust - however i did note the black manifold on one side that mounts to the exhaust ports is rusted a bit where it connects and water was dribbling out a little. That said, this has to be fresh water (not part of the closed cooling system), so while not ideal, can't see this being the overheating issue?
3. I had to top up the closed system water, and did this with fresh water (not coolant) - i put an estimated 3 or 4 litres of fresh water in. Would the lack of coolant have that significant overheating effect?
4. A few bits of the perished raw water impeller were in the pump housing - could some other bits have gone through the system and blocked it, or could it be a bit corroded and blocked? Is there any easy way to test for blockages or flush the closed cooling system?

Any other ideas gladly welcomed on what to test or inspect!

Thanks

Rob