mercury 50 1983 water problems

Hi Out there, I'm new to this so I hope for the best.
I have a 1983 (as per the serial number ) mercury 40 inline 4 cyl 2 stoke outboard. My problem is simple at idle I have strong flow through the vent / Pee hole and the motor runs well . when I hit half revs the flow falls away and I get a mix of water and steam coming from the vent. When I slow down it runs clear again. I have fitted a new impeller . ( the old one was badly worn and deformed ) this has helped but out today with the engine on-song at about 3/4 speed the problem still exists. Is there a way I can reverse flush the system other than pumping water through the pump delivery tube up into the engine as this is still the normal direction of flow.

Re: mercury 50 1983 water problems

WOULD NEED TO DDROP THE LU and blow compressed air back thru the tell tale hole. could have small bit of crud causing partial blockage restricting water flow at the higher rpms. tape a sock or rag to the water inlet pipe to catch anything that is blown out, also did you replace just the impeller or whole water pump unit, how was the condition of the cup that the impeller rides in, was it scored / worn, or in good shape'. if you dont drop the LU the crud will not be removed.

Re: mercury 50 1983 water problems

FLATHEAD MAC said:

WOULD NEED TO DDROP THE LU and blow compressed air back thru the tell tale hole. could have small bit of crud causing partial blockage restricting water flow at the higher rpms. tape a sock or rag to the water inlet pipe to catch anything that is blown out, also did you replace just the impeller or whole water pump unit, how was the condition of the cup that the impeller rides in, was it scored / worn, or in good shape'. if you dont drop the LU the crud will not be removed.

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Cheers Flathead Mac,
I dropped the LU and took it in to the service guys and had the whole pump unit serviced. I have run a water hose to the telltale and it flows free so I reassembled and went out today. The tell tale fell away at half throttle and only came back when I slowed and ran slower. When back at dock I ran the motor at high revs , out of gear, and the tell tale ran strong and clear, could i have a problem with cavitation at speed ? I have a pair of hydroplane hard rubber wings bolted to the flat portion on the motor just above the joint with the LU can these cause problems ?
Cheers
john w

Re: mercury 50 1983 water problems

I have now sorted it. I connected a hose to the tell tale and flushed it back then connected it to the pump delivery pipe.
I also removed the Hydravanes fitted to the LU. this seems to alow the boat and motor to run on the plane a little smoother,
Perhaps I/m imagining it .
Thanks for the help , know doubt I'll be on again.