The water pump could be working, but not to the optimum level...The blades on the impellar will bend, and sometimes melt and crack, over time, thus reducing it's performance...The buzzer you're hearing is a warning so that damage isn't done...

right.. but is it possible that i go to the shop, have the water pump and impeller replaced, and the overheat alarm still sounds??? what else might the problem be???.... if i can figure out where the thermostat is on this boat, i can change that at cheap cost and eliminate that as a possibility, but i can't find it anywhere

I don't know that particular powerhead well enough to tell you that...I can get the answer from my mechanic and look at your engine and we can change the impellar, but it will have to be Tuesday or Wednesday (my mechanic's shop is closed on Monday)...

It could also be a wire with a short in it coming off the buzzer...

Okay, there should be 2 thermostats...Here is the best diagram I can find to show you where they are...They are going to be at the bottom of the powerhead, but at the top of the exhaust housing (if that makes sense)...You'll have to click on the year for your engine and then the hp and finally the model...The number you are looking for in the diagram is 78...Once you zoom in, it's on the top left of the picture...I hope this helps and if you want some assistance, just let me know...

Diagram

theres lots of things that affect the pump other than the impeller, thr grommett in the cap must be in place abd in good shape, if the base plate is all dished out from wear this too can affect the pump , as i said in my pm put a new entire pump kit in not just the impeller and never crank it over dry

Since the pump is in question, I'd change it out anyway.

You can change out that water pump yourself if you're 1/2 way handy with tools. Write down you model number and serial number, then go to the dealer and buy a COMPLETE water pump KIT with the stainless steel base. Last time I priced it, it was about $50...but I'm sure it's higher now. Get a service manual too and you're good to go. Just PAY attention to the shift shaft. Read about it FIRST ! If it's like mine, that shift shaft bolt under the carbs is a pain the first time you try it. But it gets easier.

Jtexas gave a good description of the operation.

That piece of rubber hose in the intake is interesting. Are the screens still in place too? If not, there's no telling what kind of obstructions are in there.

Just because you have a good stream from the peehole on the muffs does not mean the pump is working properly. Remember the water thru the muffs is under pressure from the hose not nessessarily pressurized by the pump as would be on the water. Have and aftermarket service manual. Will be on Lake worth Sunday in the A.M. If You want to drop by. Call 817 677 5838 and I'll come in to meet at casino beach ramp.

oldfrog, actually no the filter-screen is not in place on the left side of the motor (from looking at motor from the back side); i need to install some of that screen, also; i have a roll of small metallic fencing stuff that might just work; do you think that, once i take the lower unit apart so as to install the water pump, i will be able to install the screen at the same time?

lonestar, i tried to view your diagram, but within which system will the thermostat be?
-CARBURETOR
-CRANKSHAFT AND PISTON
-CYLINDER AND CRANKCASE
-ELECTRIC STARTER PRESTOLITE MODELS MGL-4109
-ELECTRIC STARTER & SOLENOID AMERICAN BOSCH 10629-23-M03
-EXHAUST HOUSING
-FUEL PUMP
-FUEL TANK - 6 GALLON
-GEARCASE
-IGNITION SYSTEM
-INTAKE MANIFOLD
-MOTOR COVER
-POWER TILT AND TRIM
-REMOTE CONTROL????????????????????????????????????????????

the pieces of rubber are probably bits and pieces of the impeller that has come apart and worked their way through the cooling system.. never run an outboard without water to the pump.. when you run one dry the rubber impeller gets very hot and melts then sticks to the metal inside the pump... the next time you turn the engine over it tears the flippers off the impeller and starts pumping the pieces through the cooling system.. you will need a shop manual to understand the cooling system on your engine.. by all means replace the water intake screens while you are doing the water pump,both of them.. it takes less than a minute to dry run a pump and ruin it... hard starting/ poor idling on the old john/rudes is almost always carb and sync/link related if you have good spark..good luck and hope you get it going....

is this manual -- http://www.repairmanual.com/marine/2/445 -- the correct one for a 1978 Evinrude 115hp???????

Sorry, I did forget to put which one...It's in the Exhaust Housing...Part number 78...

Yes...

Correct manual. I also have the factory manual for mine, which stays in the boat. As a matter of fact, my buddy borrowed it last year and hasnt returned it...Grr ! Thanks for reminding me. As soon as I saw that front cover, I thought of it. I owe you a lunch or a beer. Thanks !

I hate to loan stuff out for this very reason ! Never again. And people wonder why I dont loan tools...sheeesh !

BTW, thermostats are inside the cylinder head covers. See the little wires going into the round holes next to the top spark plugs ? ( I havent looked at mine in a while, so I'm guessing the exact location..but that's close)

I have another little "shadetree" tip for you, once you remove the lower unit. Cut a piece of 3/4 inch plywood about 18 inches wide and 4 feet long. In one of the ends...( lengthwise)...but a "V"...large enough to SLIDE your lower unit into it. ( upright) The "V" will be about 4 inches wide at the edge and about 18 inches long at the vertex.

Make it so it will fit under the fins on the sides of the lower unit. Then slide the plywood between your trailer and your boat, just behind the wheels....so it can hold it the lower unit while you replace the waterpump. It's quick and easy.

Oh I shutter to think what's going on in this site. 1st off, that horn is an overheat horn, 2nd If your finding rubber pieces, I'll bet the farm that the impeller has come apart. 3rd. Thermostats are located in the water diverter plate below the engine block, Look for the 2 , 3/4" hoses at the bottom of the heads and where they attach to a box that is approximately 1.25" X 4.00" The thermostats and high pressure by pass valves are located in the box. The thing that has the wire coming from it mounted in the head close to the spark plug,,,,,That's the temperture sensor,,,ain't no thermostat. 4th There is still one thing that can make the engine overheat even if you install a water pump and thermostats. They are called water diverter tubes, located under the head, inside the block, between the cylinders. If these have not been replaced, You are long overdue. 5th do Not use a metal screen. That plastic screen is only about $7.00, It has a purpose, There is a divider plate in the center. That blocks the water and forces it to travel upwards to the water pump.

FYI. By this time, and as many times that you have heard the hot heat horn. You really need to run a compression test. That engine need somewhere +or- 140#. That 78 model, if it has not been rebuilt, is running high ring pistons, they don't like heat, The top of the piston gets soft (from heat) the rings pull right off, down goes the motor. The temp sensors make the horn blow off at 190 degrees. At 200 degrees you start getting piston melt down. It takes less than 15 seconds to reach 200 degrees once the horn goes off........ My advise to you is quite messing around...get it to a quilified mechanic, get it fixed. Spend the money now, it will save you big $$$$....Sorry for getting off,,,It really gets me going when I see wrong information or bad advise given out to someone that is asking for help. I've been making my living for over 20 years on outboards, and only outboards. Trust me on this one...