1977 Mercury 115 HP overheating

[dahayes]

I have a question about my Merc. 115. When running it at high rpm's say, 4000 or so, the water coming out of the pee hole is not a solid stream like it is at say, 2500 rpm. It looks more like a mist coming from it at high speed. That's not normal is it? Shouldn't it have a solid stream at high rpm's the same as it does at low rpm's? The guy I got the boat from said he just recently changed the impeller. Also, I just recently tore the motor down and replaced all the gaskets where water flows because water was getting into the # 1 cylinder and it wouldn't run at all. It runs great now, but I'm just concerned about this possible overheating issue. Any help would be appreciated!

 

[CV16]

I would buy an OEM impeller and replace it. I have had problems with Sierra more than once with their impellers. I don't know what he used, or even when. To be safe, I'd get the pump kit just for my own piece of mind.

 

[GA_Boater]

The fast you go, the less the stream looks looks solid. The wind tears it up.

Any signs of overheating?

 

[Chris1956]

The telltale should be solid at all speeds, even idle. Replace the impeller, wearplate and 2 gaskets. Inspect the pump cover. If the liner is scratched, replace that as well.

 

[dahayes]

Thanks for the advice guys! I'm not sure what brand of impeller he used. I do know it was changed within the last few months. The motor runs great and strong. I've never dealt with an engine overheating so I really don't know what to look or listen for other than what I mentioned in the initial post. The boat has no temp. gauge on it. That is something I would like to have on it for sure! As for the wind tearing up the stream, I had a boat go by me the other day with a Mercury engine on it and I could see the stream from his telltale from 200 yards away! And that dude was gettin' on it! I'm thinking mine should look very similar to his at high speed.

 

[Chris1956]

That motor does not have a thermostat. It will run dead cold at idle and a bit warmer as speed increases. As such, a temp gauge is not useful. Install a 0-15 PSI water pressure gauge and that will help you keep an eye on it.

If you have no telltale, and it is not clogged up, you will overheat. Water pump repair is $25 or so. New #3 piston (after overheat) and boring job is about $350, if you do the work yourself.

 

[dahayes]

It's not clogged up. From idle up to 2500 rpm it has a good strong jet of water coming out of the telltale. Anything beyond that and I can't tell what's going on. It goes from a good strong stream to a distorted "spitting" like look to it. It could be just distortion from the wind and waves. I don't know....

 

[Chris1956]

Has that motor been left in the water? Maybe the water intake grates are partially clogged?

 

[CV16]

I'm still betting on the impeller being the culprit. I had the same issue, buy the complete kit, housing and all and give it a try. Only use Quicksilver, Sierra has bit me with their impellers before, never again!

 

[dahayes]

Chris1956, No, the boat hasn't been stored in water. I think I'm gonna take CV16's advice and just change out the whole water pump assembly. Thanks for all the advice guys!!

 

[dahayes]

Okay guys, I took the boat back out today and I had my son with me so he could drive the boat while I checked out the telltale. I tell ya what, when that boat is running wide open (over 4,000 rpm's) the water coming out of the telltale is vaporized into steam! Without going into a lot of details, I just recently set the timing on it. It runs great now, but with the overheating issue. Is there a possibility that it isn't in "perfect" time? I set it at TDC to the distributor, but the decal on the flywheel says it's suppose to be set at .104 BTDC. I didn't know exactly what that meant or how to set it at that. Does anyone think this could be causing the overheating?

 

[Chris1956]

A 1977 Merc 115HP motor timing is as follows. Idle pickup timing 4-6* BTDC. Max spark advance 21*BTDC.

​There should be a timing decal on the flywheel. When the motor is set to TDC on the top cylinder, the timing pointer should read TDC or very close. Anything else indicates the flywheel is on wrong. Also at TDC the timing pointer on the distributor should point to the one or three dots on the flywheel.