Merc overheating problem

[ShafferNY]

Hi everyone. It's been awhile since I've posted here, but I'm hoping you guys can help me out.<br /><br />I'm working on a mid 90's, 50hp, Merc outboard that belongs to somebody I know. The guy said the horn goes off when he is trolling and it only seems to be getting about 3/4 trottle. He wanted to know if I'd take a look at it. <br /><br />He said he put a new set of plugs in it, and that didn't make any difference(I don't know what made him think it would?). He also told me that the old plugs all looked uniform, but didn't say what shape they were in(clean, fouled, etc.?)<br /><br />I just wanted to get some opinions as to what I should look at first? I'm thinking I should check the plugs(even though they're new), possibly do a compression test, and check the impeller/water pump. <br /><br />A friend said I should check the head gasket. I briefly looked the engine over and I don't see a head, let alone a head gasket. The block looks like it's all one casting. <br /><br />Anyway, I'd appreciate any and all opinions that you are willing to trough my way. Thanks

 

[WillyBWright]

Re: Merc overheating problem

Do the water pump. That way you eliminate that as a possibility right off the bat. It could be weak. It's never a Bad Idea to do a water pump.<br /><br />Nope, no head, no head gasket.<br /><br />Other possibilities include scale build-up on the cooling side of the cylinders insulating the cylinder from proper cooling, or an oversensitive overheat sender.<br /><br />Check the compression and check for good spark to all cylinders. You could get good spark at low RPMs and lose it at higher RPMs, say above 3/4 throttle. I think you have two separate problems here. My guess would be a weak water pump and a bad stator. But those are guesses.<br /><br />Connect an inductive pick-up timing light and run the motor at full throttle (in the water, please) to see if the spark is going away. If you still get regular flashing at full throttle, then the ignition system should be okay. The timing light will only flash if the sparkplug is firing.

 

[WillyBWright]

Re: Merc overheating problem

It could also be a thermostat. They only work at lower RPMs. At higher RPMs there is a poppet valve that opens up allowing full water flow. The thermostat could be scaled-up and sticking. Soaking it in vinegar to get the scale off is usually enough to get it working properly again. If that's a 3 cylinder motor, it should have one. If it has 4 cylinders, it won't.<br /><br />I'm shifting my guess here because weak water pumps usually cause problems at higher RPMs. I'm guessing Thermostat for the trolling speed Hot Horn, and Stator for no power over 3/4 throttle.

 

[ShafferNY]

Re: Merc overheating problem

I took a look at it today and here's what I found. Compression is 135 top, 137 mid and lower. <br /><br />I checked the impeller and it has all it's fins. However, it is weather checked and looks like it has lost all it's flexibility so I'm going to replace it. <br /><br />Do these motors have a horn for low oil? The owner asked me when he dropped the boat off the other day but I forgot to mention it here. <br /><br />Are the stators a common problem on these motors?<br /><br />Thanks for the help Willy. I appreciate it.

 

[ShafferNY]

Re: Merc overheating problem

I have to make a correction. I was just informed that the motor will only open up about 1/3 throttle.