1977 johnson alarm horn sounding at full speed ???

M

mk1959

Recruit
Joined
Jun 7, 2011
Messages
2
I have a 1977 johnson 115 hp outboard. Its hanging on a 1979 starcraft that I have restored. I put a bracket on the back , Enclosed the transom. Worked out OK for a novice. Problem today was at full speed the alarm horn sounded. When I reduced speed to a crawl it stopped. Motor is ejecting the cooling water. and at the dock at idle can put my hand on the heads , warm but not hot. I am having issues with a cavitation at high speed... sounds like its cavitating at high speed. could this cause a decrease in the influent water supply to lower unit ????

Any ideas,

Mark West Virginia
 
Bosunsmate

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,135
Re: 1977 johnson alarm horn sounding at full speed ???

Yep it could do, Hows the telltale at high speed? Maybe your bracket has the motor set at the wrong height.
Ventilation is when the motors so high the prop sucks air from the surface.
If you post a photo of it in the lowered position and a close up of the prop im sure someone will comment on the height setting
 
E

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: 1977 johnson alarm horn sounding at full speed ???

Agree. Possible that the added transom bracket may position the engine too high on the boat when at higher planing speeds. A high mounting may allow the top two holes (of the four lower unit water intake holes) to actually be above the water line flowing from under the boat at WOT. When those top two holes get exposed, it's easier for the water to flow out of those holes instead of going up to the water pump. That may the the cause of the overheat warning. If you had a water pressure guage plumbed into the cooling system, you may find the pressure has dropped below 10 lbs at higher speeds-not enough to keep the powerhead cool.
 
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