1974 Mercury 850 thunderboalt please help!

rescue95095

Cadet
Joined
May 4, 2013
Messages
6
OK as some may know I am brand new to owning a boat and having some problems. I have a 1974 mercury 850 thunderboalt 85 HP outboard motor on my 1977 Champion Bass boat. I had the boat out several times and starts but is very hard to start and runs my battery dead. The motor runs good at high idle and boggs down at full throttle. If im idleing it will shut down also. I dont know if this is a four cylander but i think its a 2 stroke? I replaced the primer bulb & my uncle said this needs to be full and hard to squeeze and it will start every time. And that was true but now it wont pull gas in the bulb any more there for I can not even start the engine now. I guess i need from reading to change the water pump, fuel pump and clean and possibly rebuild carburator?? I do not want to purchase the wrong ones and dont want to buy to much if I dont need a re build kit.

Any one that can tell me more about my basig engine and parts where i can buy and get the right ones i would appreciate it. I changed the lower unit gear lube and the primer bulb and it runs on 50:1 is all my uncle told me. I have no clue about carbs either but want to do all on my own so any help with those id appreciate. Spark plugs and wires have not been changed either.

Another problem I have is on my mercruiser throttle it will not go middle speed I have to slam it all the way down to get it to go fast or i can make it run faster with the high idle lever, I dont understand why it is like this.

And I have a power trim and tilt that is mounted by my battery unaware of model or make but this thing leaks I replaced the O rings and it still leaks to where i have to keep putting in 5w -30 oil to get it to go up and down I know I have lots of problems and Im frustrated and would appreciate the comments. Thank you all
 

waflyboy

Cadet
Joined
Aug 29, 2012
Messages
24
Re: 1974 Mercury 850 thunderboalt please help!

Howdy,

I think you're right to start with a rebuild of the carbs and fuel pumps. To find the right parts, it might help to locate the outboard's serial number. (It's probably stamped on the back of the mounting bracket that holds the motor to the back of the boat.) Then go to Mercury Outboard Parts - iboats and use the serial # to help you find the right kit(s).

If you don't have a Seloc manual for the motor, you might consider buying one. If you're unfamiliar with working on outboards, the book will likely save you a lot of time. I'd never worked on a boat motor until last summer when I rebuilt the carbs and fuel pumps on my '76 Merc 500 (50hp) with the help of the Seloc book and the folk on this site. She purrs like a happy kitty now.

Never done an impeller job, but planning on doing that in a few weeks!
 
Top