1973 mercury thunderbolt 850

[Kmyers]

I have bought a boat with a 1973 mercury thunderbolt 850 that has been sitting in a barn for about 20 years. I did a compression test on it, cold of course and I got the readings 135 cylinder 1 142 on cylinder 2 120 on cylinder 3 and 145 on cylinder 4. I am trying to find out if the lower reading on cylinder 3 is going to give me problems later. I got a good deal on the boat and the motor is a bonus but need to know if i should put some money into it. I know I will need to buy an engine harness, carb kits, water pump, rebuild fuel pump. Any help will be greatly appreciated.

 

[Maxz695]

Re: 1973 mercury thunderbolt 850

Remove the plugs and spray some fuel mixture in the cylinders through the plug holes. Turn the engine a few times and then try taking the comp again with the plugs out. The cylinder that is weak may have just been dry after sitting so long.

 

[mr 88]

Re: 1973 mercury thunderbolt 850

I would turn the motor as horizontal as possible and soak the heck out of that #3 and of course throw some in the other holes as well.Do it for a few days. The rings ay be semi stuck after sitting for so long and direct lube is much better than hoping the oil from the gas frees things up.If you consider the motor free then just run it until she loses power or idles badly. I doubt it will throw a rod or,seize up .Mr Marvel will help your compression number, I think.

 

[Kmyers]

Re: 1973 mercury thunderbolt 850

Thanks guys I will start soaking all cylinder right away. I guess I should not have been turning the motor over without having an fuel for lubrication but the fly wheel turned so easy I couldn't resist

 

[Maxz695]

Re: 1973 mercury thunderbolt 850

I don,t think any damage was done with just a few cranks to test compression. The lube will create a proper seal and give you more acurate readings. Being it sat so long in one posistion the cylinder with the low copression was probably the one exposed to the air most through the exhaust port by being all the way on the down stroke to the lowest position drying up any oil in the cylinder over ten years time. Should be fine with the other psi numbers you threw out there

 

[hotrod53]

Re: 1973 mercury thunderbolt 850

I would just pour some Marvel MO or even 2 stroke oil in it and roll it over by hand, make sure your direction of rotation is correct or you'll trash the impeller being that old. Mine sat for 12-13 years and it fired right up. I would just be sure that she pees right away so it doesn't burn down on you during test fire. In 20 years, the rubber impeller is probably shot and may not pump even for a test fire.

Harnesses are a few bucks, like $200, the rest is cheap. With that kind of compression after sitting that long, its probably a good motor. I wouldn't put much stock into an accurate reading until after it's run at least once.

 

[mr 88]

Re: 1973 mercury thunderbolt 850

In many if not most cases there is enough wire that you can splice the new[correct]size wire and connectors into and by-pass buying a new harness.Nice call on the impeller HotRod,often number 1 major as in $$$mistake of new owners of marine engines.A manual if things look good is also a nice addition to the library.

 

[tater mcdonald]

I have the same motor and blew it up last weekend put another on this week and now the lower unit wont go into gear any thoughts on what i could have done wrong

 

[GA_Boater]

I have the same motor and blew it up last weekend put another on this week and now the lower unit wont go into gear any thoughts on what i could have done wrong

What did you do wrong? I don't know about the motor, but you hijacked this old, dead thread with an unrelated question. Please start a new topic and please don't reply to topics inactive for 90 or more days. Also add some more detail - Like what all did you do to the motor. Thanks.