how do i make sure this 85hp merc is water ready?

[gijordan07]

hey there, first time boat owner, and just itchin to get out on the lake.

I have some automobile experience, but never an outboard. The motor is a 1974 Mercury 850 with Thunderbolt ignition. I've been told that it runs by the guy who sold it to me, and told by a friend that it will rotate, but a cylinder has low compression. I don't know that either of these things are true or not. I've also been told that "low" compression is relative too, and depending on the difference between cylinders it may be fine.

So here is my question, where should I start? I have a cylinder compression tester for a car, will that work the same? how do I bump the engine over without it starting? Will I need a trash can full of water or some ear-muffs and a hose just for this test? Also, the control unit was disassembled by "said friend" before I got it back, I have all the parts, just a little unsure of where a few of the components hook up.

If anyone can help me out that would be fantastic!!
Thanks,
Jordan

 

[Chris1956]

Re: how do i make sure this 85hp merc is water ready?

Do take a compression test. Remove the spark plugs (bottom plug can be removed with a open end wrench). Jumper the starter solenoid, located on the starboard side of the motor to make the starter spin, key off. Compression should be within 15% between all cylinders.

Change the gear oil, using a qt of Marine Gear oil for mechanical shift motors.

A flushing device is available for a few $. Connect a hose to it for anuy on-land testing. Fuel mix is 50::1 regular gasoline to TCW-III 2 cycle oil. Pump some grease into the fittings.

 

[gijordan07]

Re: how do i make sure this 85hp merc is water ready?

cylinder compression is as follows: #1- 110; #2- 115; #3- 110; #4- 90; the last one i'm not quite sure about...the solenoid shorted out/battery died during testing on the fourth cylinder...i'm not sure it cranked quite as long as the other cylinders...made me nervous to crank it too long also, just a few seconds each. really itchin to get out on the lake, is it possible to assume its probably okay, and just make sure to put some time/money into it this winter? i dont want to destroy it either tho..having a hard enough time finding wires, solenoid, etc..

Could you also tell me where i change the gear oil at? I've also heard a lot of talk about impellers..is this same thing as water pump? should this be done before hittin the water too?

thanks for the help!!

 

[aerobat]

Re: how do i make sure this 85hp merc is water ready?

with these results on a 1974 motor just fire it up and go boating, all seems good. due to gear oil change - you have one screw on the bottom and one higher. unscrew both, drain the oil and fill new marine gear oil via a gear oil pump via the lower screw until it comes out of the top screw. wait a little, refill and tighten both screws. the engine will be a runner !

 

[gijordan07]

Re: how do i make sure this 85hp merc is water ready?

Spoke to the local marina today about some plugs, wires, and a solenoid...mentioned the compression and the guy said 140 is where the cylinder compression should be, even if all 4 were pushing 115 it probably has a bad powerhead...any truth to that? If it does need work that's okay, but I want to know before I make things worse just runnin it...

 

[swamp-fox]

Re: how do i make sure this 85hp merc is water ready?

Maybe on a new motor should have 140 but not a 37 year old! Mine are 115-120 and it's perfect. You may want to check the 90 again that seems right on the boarder. Everyone one here says 10% variance and as long as they are the same no matter if its around 100 or 140.

 

[charlie741]

Re: how do i make sure this 85hp merc is water ready?

I would change the impeller/water pump before taking to the water. If you search threads you find dozens on the how to, its not hard. Also, I learned how by watching some you tube videos that gave step by step instructions. You should change every other year at least, but I'll do mine every year. I wouldn't worry about compression you got until you've seen how it does on the water.
If you still need help with the controller, post some pics and people on here are loaded with tips and advice to get you back together again.

 

[Mi duckdown]

Re: how do i make sure this 85hp merc is water ready?

Impellar is a must. All your other work will be fornot if you overheat.

 

[Wingedwheel]

Re: how do i make sure this 85hp merc is water ready?

Ditto on the impeller! If the motor hasn't been run in awhile, perhaps a small squirt of oil in the cylinders will help bring the compression up. I had the same motor and it was a great runner. I would recommend rebuilding the carbs, or at least a good cleaning and a new fuel pump diaphram. I would also recommend new floats as they tend to swell a bit with age and stick. The wires should be checked for bad insulation which was a problem with some older Mercs. Your compression sounds ok as far as getting it to run and if cared for right, you will get many hours of fun on the water. As with any older motor you will have to spend some money to get it to a point where you feel totally comfortable with it. Last tip, buy a good manual. That will save many hours of headache if there is a problem. Good luck and welcome to boating!