Mercury 800...questions

Hey everyone,
Im brand new to owning boats and have a few questions regarding my motor.
Trust me, i feel stupid asking some of these questions since im pretty decent with a wrench.


1) Using the serial number (3056287), I believe my motor is from 1971.
But, it looks like a late 80's model because it is all black. Not white with the silver part on it. Did i miss something?

2)Supposedly, it needs a starter. The guy said it wouldnt start out on the lake for no reason. I noticed the wiring is TERRIBLE. could this be the problem? I hope he didnt screw me over and the motor is actually shot. heres a pic of just some of the bad wiring.
http://s95.photobucket.com/albums/l121/rtpassini/?action=view&current=BOAT007.jpg

3) Where are the drain plugs for the lower? i need to put oil in it for over winter. Is there anything else i should do over winter besides change out the oil in the lower?

4)Spark plugs. These dont look like normal ones. What kind do i need? heres a pic of the 2nd plug.
http://s95.photobucket.com/albums/l121/rtpassini/?action=view&current=BOAT002.jpg

5) While looking around the top of the motor, i found a bunch of leaves and "helicopter" things (brown colors under the dist. cap) Should i just clean them out best i can? is that bad they are in there?
http://s95.photobucket.com/albums/l121/rtpassini/?action=view&current=BOAT006.jpg


Again, thanks for all the help. i know i asked a lot.

Re: Mercury 800...questions

Your motor is late 60's early 70's.

The original wiring on those motors deteriorate badly and will crumble until replaced. Wiring harnesses are available.

The drain and vent holes for the lower unit are in their normal locations. One on the bottom just to the side of middle towards the front (if mounted on boat) and the vent up towards the anti-cavitation plate on the side of the lower unit section. The have large flat screwdriver slotted heads.

Those are the (surface gap) type spark plugs used with that type of electronic ignition.

You probably should remove any "rat's nest" looking piles of leaves or other debris.

Re: Mercury 800...questions

thanks!! i appreciate the quick response

is there any sites online you can order parts from?
like with a microfiche?

Re: Mercury 800...questions

Have the same motor as you. Agree that yours is a 71 or 72 model year. There will be RED accent color bands on the cowling.

You will need to repair/replace all of the wiring harness in the motor. Complete new harnesses are available if you shop around. I repaired mine with great results.

The no-start could be for any number of reasons. You need to perform a compression test and ignition test to get a picture of the condition of the motor. These two things will tell you if the motor may run or not. Search these forums for instructions on how to perform these tests.

There are several on-line parts suppliers that have good exploded view diagrams.....

mercruiserparts dot com
dougrussell dot com
ishopmarine dot com
crowleymarine dot com

...... to name a few.

Re: Mercury 800...questions

ill do a compression test in the spring time.
i just put the oil in the lower and some fogger in the cylinders. it turned pretty easily, so at least i know its not locked up! haha

for cars and bikes, theres repair manuals you can download for free online...does anyone have access to these for this motor? or should i buy a manual.

ill probably just build a harness, ive re-wired motors in my cars before due to different engines being put in. i just need a diagram.

Re: Mercury 800...questions

Like you and Moody Blue, I too have a '69 version of the same motor. As a matter of fact, MB and I have conversed about them. The wiring you show, is typical for these motors. A problem I've noticed on quite a few is, the red and brown wires to the switch box get pinched by the forward cowl support. Once the insulation is cut, deterioration will soon follow.
If you want to make your own harness, make sure you use a marine grade wire. It is different than automotive wire. If it were me, I'd get a new harness and just replace the whole thing. Much easier. Splicing into the main "plug" might be a bit tricky.

Re: Mercury 800...questions

once it gets warm out, ill take a look at it more. but the wiring in that boat is a rats nest. the guy before me had no clue what he was doing. chances are ill end up redoing the whole thing.

this boat is simple. its old, so not a lot of "extras"

just glancing, the wiring on these motors and everything seems pretty simple. at least compared to the motors ive done on cars.

thanks for the heads up about marine wire, that i didnt know.

Re: Mercury 800...questions

I came up with 1971.
Plugs are normal.
I run NGK BUHW in mine...they do not have the typical hook electrode.
The internal wiring harness is going to set you back over $100. (The external one is twice as much).

PS - don't feel stupid. I've been turning wrenches for almost 4 decades...it's impossible to know everything. Before today, I didn't know how to work on a washing machine!

Re: Mercury 800...questions

It that motor has good compression (90PSI+ and even on all cylinders), you might fix it up. I would use some jumper cables directly to the starter to spin the motor and check the compression. If that works, obviously the starter is good, and it the compression checks OK.

Replace the spark plugs. They are avail at most auto parts places. Chanpion or NGK are good. Get the correct ones, usually either L-76V or L-78V Champs.

The wiring harness is avail at Surplusunlimited.com, plug wires should be checked for spark leakage. They unscrew from the dissy cap and are avail at oldmercs.com.

There is a ignition test avail on this forum. Do a search for 'Distributor powered CDI Test".

There is a link and sync on this forum. Search for "Synchronizing Merc Inlines"