mercury Viking problems

[Lucky]

Hi all you knowledgable forum members,

I need advise on a strange behaviour of my Merc OB. It is aViking 10HP upgraded to 15HP. I had a problem last fall when me and the missus was out on a trip to town. We where going great and entered into the town and I pulled back the trottle when entering the habour. Then suddenly white smoke or steam came out of the exhaust!

Started it up again and it went on for just a few seconds and the same thing happened again! Got the motor off the dink and started looking at what could be wrong, no appearant issues to be seen. I spoke to a OB mechanic who knows Mercs and got the advise that it's either the thermostat or the impeller that has failed.
I mentioned that I believed the head gasket had blown, silly me, 2 strokes don't have them! I did the fix as advised and tried to start the engine just a few days ago. And what happened, the same thing again! The motor would run using start gas but not on petrol!

Could there be a cooling water ingress into the power head? This is a bit of a mystery to me.

Anyone with an opinion as to what this might be is welcome with their advis. It would be very much appreciated.

Looking forward to hear from all you clever people on the CF.

Thanks from Lucky

Happy lead free sailing