1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system issues

[SD_Redneck]

I have a Mercury 850 4 cylinder with the Thunderbolt ignition system that is overheating. I was out all day fishing at the local lake driving all over with no problems. After I dropped my friends off at shore, I headed to the boat dock at full power when the motor died at full speed. I initially thought the battery for the motor was too drained so I used the trolling motor to get to the dock. I took it back out the next weekend with a full charged battery and took off from the boat dock. I idled out of the launch area with no problems and got up to speed. I drove a few hundred yards when it lost power and died again. I put the boat back on the trailer and went home. When I got there I noticed the telltale was plugged so I ran a piece of wire about a foot into the line to clear it. I then hooked up a flush attachment and it pushed clean water out of the telltale. I then put the motor in a 55 gallon drum full of water and started the motor. While the motor was running, I watched the telltale and it spit out a few drops of brown water and a consistent stream of exhaust appearing gas. After the motor had been running for about 5 to 10 minutes, it started slowing down and died. When i was feeling the motor to detect heat, the top half was warm but not too hot and the main part of the case directly above the water line was extremely hot. I let the motor sit overnight and flushed it out today and it had a solid stream of clear water coming out of the telltale. I also noticed there was a slight bit of water coming out of the number 1 cylinder from the spark plug.

The vin number is 3493XXX.

Any ideas would be greatly appreciated.

 

[bktheking]

Re: 1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system is

These 85's from the 70's are known for cooling issues, water getting into the cylinders. Check your plugs and see if they are water washed.

 

[SD_Redneck]

Re: 1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system is

I took the plugs out and they look fine. Was only a slight trickle of water coming out. I didn't see any corrosion or rust on the plugs either. The only thing on the head of the plugs was a slight carbon buildup but nothing that would cause a problem.

 

[dwco5051]

Re: 1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system is

I took the plugs out and they look fine. Was only a slight trickle of water coming out. I didn't see any corrosion or rust on the plugs either. The only thing on the head of the plugs was a slight carbon buildup but nothing that would cause a problem.

This leakage around the plugs could be the cylinder block cover gasket leaking by. This may be a simple fix providing the over heating did not warp the block or cover. With the cover off you can also check for crud build up in the water jacket. The problem with this design it is very hard to see if any scoring of the cylinder walls happened due to the over heating by just looking down the spark plug holes without some fancy inspection device.

I will have to add that replacing the block cover is only simple if all the bolts come out without breaking.

I had a 1976 model of that engine and put over 3500 hours on it before it burnt a piston. Yours appears to be a 1973 by the serial number, I think that was the first year of production for the 850.

 

[Faztbullet]

Re: 1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system is

Replace the impeller then repair the water leaks...

 

[dwco5051]

Re: 1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system is

Replace the impeller then repair the water leaks...

Agree with that. A little leaking around the cover by the spark plug is not the end of the day. Another time running without proper water flow might be the end of the motor.

 

[emckelvy]

Re: 1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system is

Do a compression check before anything else. With the powerhead having overheated multiple times, it's very likely to have fried the pistons/rings.

If compression readings are still OK, pull the lower unit and inspect/replace the impeller as required. With the lower unit removed, run water up the copper water supply tube to check for circulation thru the cooling system.

Then, if all else checks out, fix any water leaks you find.

No sense throwing good $$$ and labor at the motor if the powerhead is fried.

Note that when you do run the motor, a good way to tell if there is adequate water flow thru the powerhead is to feel the exhaust manifold cover (big flat cover, Port side). The cover should be cool to the touch or lukewarm at most.

If it's very hot, you don't have enough water flowing thru the powerhead, and exhaust is rapidly heating up the cover.

HTH & G'luck........ed

 

[SD_Redneck]

Re: 1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system is

Mallory Water Pump Kit, complete 9-48309 - iboats
I am thinking that this set is what I would probably need to fix the problem. Any other ideas? No local boat shops that know anything around here.

 

[Chris1956]

Re: 1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system is

That appears to be the correct waterpump kit. Looks to have the pump base as well. You need to clean the driveshaft well with sandpaper and then grease it to avoid tearing the new driveshaft seal in the pump base. There is a gasket under the base, as well as an oring. Grease both before installing. There are gaskets on both sides of the wear plate. Install the impeller and key. Lubricate the impeller and while turning the driveshaft CW, install the pump cover.

Put some silicone on the leak on the cylinder head water jacket.

BTW - a compression test firstest is the best idea. You can inspect the cylinders, by removing the transfer covers.

 

[SD_Redneck]

Re: 1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system is

Ok thanks for the advice. One problem, I do not know how to test the compression or where to get a tester or the numbers I should see to know its good. A little mini guide would be extremely helpful.

 

[Chris1956]

Re: 1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system is

Compression testers are real common. Get one at any auto parts store. Remove spark plugs. Insert compression tester into top spark plug hole. Jumper starter solenoid so starter will crank motor. Watch compression gauge. When it stops rising stop cranking and note numbers. Repeat for all cylinders. As long as compression is close on all cylinders, motor is good. If you fried a ring, you will have zero or maybe 5 PSI on that cylinder. normally, good compresion is 90PSI or more.

 

[SD_Redneck]

Re: 1970s (I believe) mercury 850 with thunderbolt ignition. Having cooling system is

Great thanks for the help!