trouble with 1978 yamaha 28hp

[timbo]

Hi needing some help with my outboard. it is a 1978 28hp yamaha.
Was out one day cruising along and the motor overheated because of weed covering the intake. Noticed when the sound of the motor changed due to overheating and then stopped the motor and freed the weed but it has never been the same. Has been overheating and some times lots of water comes out the telltale and sometimes none at all. I took the gearbox off and attached garden hose to the water inlet pipe (not sure if this is bad or not) and water was flowing freely. started her up was running ok. replaced the impellor and took her out again, still had the same problem. would pump water out the tell tale then just trickle then none etc. had a look at the thermostat and has already been removed.
Have I damaged the motor by overheating?
If anyone has any ideas ... would be greatly appreciated.
thanks
Tim

[Clive]

I am no expert but it sounds as if the impela has been damaged by the over heating, I would replace ut and see if that fixes the problem. I was told that if the motor can be damaged in a ver short time when its over heating.

[timbo]

I replaced the impellor already and when running with earmuffs on seemed ok. put it in the water and was having the same problem.
thanks for your reply

[ressiv]

perhaps youre water-channel by the inlet inside the motor is blocked???

[mudrunner]

Timbo,
Did you replace the entire water pump kit or just the impeller. If the wear plate under the impeller is groved, you lose efficiency. If it ran dry during the time it was clogged by weeds, it doesn't take log to damage the impeller. You got some good suggestions. It may have seemed OK on the earmuffs because of the water pressure being delivered by it. It's worth looking at it again. While you have the lower unit down, run a wire up the cooling pipe to the head. I've done that and dislodged some caked mud. Good luck.

[Ken]

timbo,
I am not familiar with your particular model Yamaha, but here's my .02 worth:
The only way to tell if you have damaged the motor is to do a compression test on the cylinders.
It sounds like you have decent water pressure with the new impeller, but there are two other items invloved in the cooling - the thermostat and the pressure control valve (if your motor has a PCV).
You really need to install a (new) thermostat - without one, sometimes the top cylinder water jacket will not completely flood, causing uneven cooling, which can lead to cracked rings and distorted cylinders.
If your motor has a PCV (aka pressure relief valve, poppit valve) you need to remove it and inspect and clean of any debris. The PCV is designed to open when the cooling system pressure gets too high (more than 20 psi), but sometimes debris, mineral deposits will clog it and cause it to stick open - when this happens, cooling water will prematurely dump out the relief holes w/o cooling the motor.
The cooling-water pilot hole (aka side discharge, tell-tail, pee stream) can get clogged with sand/shell and not give a full discharge. Remove the hose from the nipple that plugs into the lower cowling and clean out the nipple - this is where debris gets hung up.
Do you have an overheat alarm on that motor? How about a temp gauge or water pressure gauge?
The best indicator of water pump performance is
a pressure gauge.
When you run the boat now in the water, what symptoms are you having with the motor?
Ken K [img]smile.gif[/img]

[ressiv]

ken:
the 28A (originaly 25B)is an ordinairy simple 2stroke motor..no special gadgets on it....