1978 Mercury 140 TOP Overheating

[stephenspann27]

This motor has been setting off the overheat alarm sooner and sooner. I can only go about 5 minutes at cruise before it overheats. I took the water jacket cover off and didn't find any heavy corrosion or blockage. The water pump is new, and I get a strong stream out of the pisser at all times. I read that on some outboards you can have gasket failures at the powerhead/mid section that allows hot exhaust to heat up and pressurize the incoming water. I am seeing steam rolling off the midsection and I'm wondering if that's my problem. I've never had this motor apart, so I'm not sure where the copper tube ends. This motor has no thermostat or poppet valve to my knowledge. I am close to just parting this motor out.

[milkdud]

You have an overheat alarm on a 1978 140? Didn't know they had one......

Have you actually put your hand on the powerhead to feel if it is actually hot? Is it bogging down like its hot or is the alarm just going off??

Does this motor happen to have the AUTO BLEND oil premixer in used? That could possibly have an alarm in it if the oil is low.


If it truly is hot then it can be a host of things. But lets make sure it truly is getting hot first.


C

[stephenspann27]

I added the overheat alarm. There was a pipe plug in the cylinder block cover that you can remove an insert a sensor. The motor is truly getting hot. The water coming out of the pisser is blazing hot, I can also see steam coming off of the mid section. The motor will also start to run rough as it gets hot. This is a premix motor. The overheating is consistent, if I run above 19mph for over 5 minutes it will start to overheat, then if I slow down and troll a while the buzzer will go off. I know water is circulating well, and pressure is good. I'm thinking there has to be exhaust getting into the water, or some major carbon build up in the exhaust area, or on the pistons themselves. I ran it out of gas a month or so ago, and it started clattering really bad and sort of "ran away' on carbon. I have put so much time an money into this motor it is unreal. I have decided to pull it off my boat, my only motive to fix it is so I can sell it whole instead of for parts. The motor went from overheating once every few weekends, which was usually after a long full speed run, then shutting the motor off without letting it idle a minute, then to overheating once every other weekend, to now overheating constantly.

[Dave Strong]

Exhaust plugged? Getting lots of water coming out of the exhaust reliefs in the mid section?

Dave

[stephenspann27]

Dave, I hadn't thought about that. Where do you think it would be plugged? The ports in the midsection? It's a shame this motor doesn't have a removable head, I'd like to look at the top of the pistons. If I take the exhaust plate off can I see them through a port or something? I'm not that famliar with crossflow 2Ts.

[milkdud]

If this is a 1978 140 then it should have a distributor. That makes it pretty easy to pop off the center transfer port cover and look at the pistons if you want to. You will get to see the intake side. Also at that point you can shine a light into the cyl and look thru the spark plug hole. cyl 3/4 cover is pretty easy to get off. 1/2 and 5/6 are much harder because they are covered with stuff.


Now the powerhead lifts off when you taker he nuts off the 8 powerhead bolts under the beauty valance.

I think the pisser on that motor comes from the top of the block right? Put a clear hose on it and run it and watch it dump out. If its got good flow out a 1/4 inch hose without bubbles then Im not sure.

But if you see lots of bubbles or you the flow is not very good then I bet you have a restriction or bad gasket somewhere.

The pisser in its stock form restricts down to like 1/8th inch so even low flow will look good because its forced out such a small hole.


What is very common is when the old impeller pump looses a few blades and they migrate up to the powerhead and block water entering the powerhead. The block can still fill up with water, but not enough flow to cool it.

The water tube can be incorrectly installed dumping water flow.

The impeller itself can be junk. Ive seen some spin on the hub.

Does it take awhile before the water comes out the pisser, or when first started does it come out within a few seconds?

C

[stephenspann27]

Lot's of info there Milkdud, thanks. It does have a distributor. I've been hesitant to remove the exhaust plates because I know several bolts will snap off. I snapped 4 off when removing the head plate. I have run this motor into the mud and sand several times, I run my boat in a lot of shallow areas. When I had the cylinder plate off, there was a small amount of sand in there. The pisser hose comes off the top of the block, so the whole block has to be full and pressurized for it to pee. I am getting a really strong stream. The impellar I removed at the start of the season had not fallen apart, it was just really worn. With the new impellar I get a stream a lot sooner, maybe 5 seconds. The old impellar barley put anything out at idle at all, and the motor did not seem to run hot back then (I didn't have a working alarm at the time so it's hard to know). The cylinder plate started leaking when I put the new impellar in, I guess it is a little warped. The leaks were very small. As the overheating started getting worse, I figured I better pull the plate and look for corrosion, then take the opportunity to replace the gasket and fix the leaks at the plate. I ended up having to use a bunch of RTV on each side of the new gasket in order to get a seal around the spark plug area. Now that the plate is not leaking, I didn't find any corrosion, etc, I am scratching my head. I have had the lower unit off three times with no changes to the overheating.
I know a lot of people say that running your motor too high can cause over heating. On one hand, this seems like my issue, because I am running a 20'' shaft motor, on a boat designed for a 25''. However, I am using a 9'' static setback plate, mounted really low on the transom to get the motor down to what I think is the proper height. I have hung off the back of the boat while on plane and looked at where the AV plate is riding, it is just above the surface. The pisser stream to steady and strong while on plate. I hadn't thought about there being air in it thought. It just doesn't make sense that it wasn't overheating before, it is now, and haven't changed the height of the motor. Everytime something goes wrong on this motor I think, "I'll just fix this one other issue.. then it'll be fine." then something else goes wrong. I am tempted to finally draw the line and cut my losses.

[stephenspann27]

Here are some photos of the cylinder plate. The corroded are I have the pic stuck in goes nowhere. I cleaned all of the corrosion off the plate.

[milkdud]

Im talking about opposite side from the exhaust side. There are 3 transfer/intake covers that have fins on them. They usually come off pretty easy if you want to look in the bore/piston there on the intake side of the pistons.

When you look at the spark plugs after running did you see any hint of moisture? Were any of the spark plugs super clean vs having some carbon build up on them? If it has an internal water leak on the exhaust plate usually it will steam clean that cyl and sometimes it can cause overheating. If your plugs look the same and no water droplets or moisture present I would not take the exhaust cover off. It can be a pain.

It sounds like your looking at everything.

If you have good water flow, I would start to wonder about your timing and fuel mix. If its leaning out some how it will get warm and if its got too much advance it will heat up if your lugging it.

The overheating is consistent, if I run above 19mph for over 5 minutes it will start to overheat, then if I slow down and troll a while the buzzer will go off.

I am running a 20'' shaft motor, on a boat designed for a 25''

If it cools fine when not planed out, but overheats running fast you may be too high... Do you have any pictures of it on the boat showing prop height.

C

[stephenspann27]

I have actually seen what looks like moisture on the plugs. Last year I plugged this motor up with silt and I'm wondering if I warped something badly then (it got hot), and the problem has gotten progressively worse. I haven't looked really hard at any of the plugs, but I know none of them have looked very carboned up, but I wouldn't say any of them looked excessively clean either. I am looking for a side profile photo of the boat with the motor trimmed down. I tried to follow the rule of using a straight edge off the bottom of the boat, and measureing up to the AV plate. I think it' son the high side but I don't think it's bad enough to be causing these problems. Speaking timing, someone had the motor maximum advanced. It was at 24 degrees at idle. It's supposed to be at 4 degrees. I retarted the timing and that made the engine run a million times better. This was before I had the overheating issues. Pics in next post.

[stephenspann27]

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[stephenspann27]

I believe these are the transfer port covers you are talking about.

[milkdud]

Best I can tell form that angles is your height look ok ish. I would not think its your problem, but every boat throws water differently. If you got water coming out your tell tail at all times then I will assume it not too high.

Now if you have noticed water droplets on your plugs, then you got a problem. If its showing up you either have a inner exhaust cover leak, or a bottom main crank seal leak. Can you remember what cyl or cyls the water showed up on?

Now that seems to be a pretty big boat! What pitch its that prop? What RPM are you getting at WOT?

C

[stephenspann27]

Agreed, my current boat is too big and heavy for this motor. My parents owned a 1977 Chrysler Funster since 1979. The Chrysler 105 gave up somewhere in the late 1990s. My dad bought a deck boat with this 140hp merc on it and swapped it onto the funster. I brought the boat out of mothballs in around 2003 after it sat for 4 or 5 years. I did some work to the hull and cleaned the carbs. After I moved out of the house and went to College, my dad sold the boat to a friend. That friend took the boat to a mechanic for a "tune up." The mechanic replaced some bad floats, which was good, but he also adjusted the the timing while on the water, with no timing light, which is why it was max advanced. The motor quit on my dad's friend one day and he just gave the whole boat back to my dad for free. My dad replaced the coil and got it running. I brought the boat down to Katy, Tx with me last year and ran it trouble free. The old funster was falling apart though. transom rotted, floor rotted. I found my 1998 19' Sunbird Fish and Ski on Craigslist. No motor, no upholstery. I paid $900 for it. A tire blew out on the trailer taking it home.. so $300 on new wheels and tires. Then I replaced the leaf springs. I replaced all the upholstery. I quickly found out that my motor was to short for the boat so I bought the setback plate and drilled new holes ($350). While I had the mercury on the stand, I replaced the timing belt (that runs the disty), the water pump, and all the seals in the lower unit. The motor really struggled to get this big boat on plane, fixing the timing helped a great deal. I'm running a 17p prop right now, and with 6 people in the boat it's hard to get on plane. I can run 33mph, no sure what RPM, I don't have a working tach. If I were to keep this motor I would drop down to a 15p prop. I also lost reverse a couple weekends after fitting the motor to my new boat. I ordered a new sliding gear dog, and a new reverse gear. When i took the gear case apart I found all kinds of carnage. A piece of the shift cam sheered off and floated around and damaged the pinion gear badly. I replaced a lot of stuff in the lower unit, and I'm good there now. I also had to replaced two of the carb floats. Is anyone still reading this? :-P Sadly I am leaving a lot out.

[stephenspann27]

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