Compression test on 1973 mercury thunderbolt 850

K

Kmyers

Seaman
Joined
Nov 23, 2012
Messages
59
I bought a 1973 mercury thunderbolt 850 and that has been sitting for about 20 years in a barn. I did a compression test on it cold and it shows 135 cyl 1 142 cyl 2 120 cyl 3 145 cyl 4 is the reading on 3 going to give me problems. I am trying to decide how much to put into it I got a pretty good deal on boat and motor was a bonus. So far I know I need an engine harness at least any help is greatly appreciated.
 
M

Maxz695

Guest
Re: Compression test on 1973 mercury thunderbolt 850

If you post the serial number of the engine someone can guide you to the part number for the harness and you can search ebay for it under the part number. I doubt highly it is not an obsolete part on most new parts sites.
 
K

Kmyers

Seaman
Joined
Nov 23, 2012
Messages
59
Re: Compression test on 1973 mercury thunderbolt 850

Thanks maxz695 I have found the harness on a couple of sites and I will check eBay for the part number also but I want to make sure the motor is worth putting some money into.
 
JB

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Compression test on 1973 mercury thunderbolt 850

Don't worry about the compression until after you have gotten her running. Then do a decarb and a new test.
 
sutor623

sutor623

Rear Admiral
Joined
May 23, 2011
Messages
4,089
Re: Compression test on 1973 mercury thunderbolt 850

Do you need internal or external wire harness? I haven't looked through these but possibly one will work for you? At least it will give you an idea of price on these older harnesses. How does the lower unit feel? If I were you I would yank it loose and see if it even comes loose, then check if it shifts. So many headaches could pop up on these motors. When I got mine up and running, it was literally part after part until I got a reliable motor out of her. Started with lower unit rebuild, then starter, then trigger plate, plug wires, carbs, fuel lines, wire harness, stator re-wire, yada yada. If I had known I may have taken that $4-500 and just put it toward a newer motor. Dont get me wrong, now that shes runnin Im happy, but boy what a frustrating journey......

http://store.oldmercs.com/SearchResults.asp?Search=wire+harness
 
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