Completely confused by 1976 Mercury 850

[MrMichael]

For a few years I've had trouble with sudden accelleration (like trying to pull up a skier) It would just die. As the years have gone on it has needed to be nursed up to speed more and more. I've tried taking it to mechanics in the area but no one is willing to work on a motor this old if they haven't been working on it right along.

This year it started like a dream on the first try. I spent one day on the lake with it. It ran a little better than usual. The next day it would crank and crank but would not start. I had to get in the water and man-handle the boat from the dock to the launch.

Since then I have tried and tried to trouble shoot it with little to bad luck.

I am getting spark.

I have pulled the brass screws from the front of the carborators and have been able to pump gas out.

I have blown air through the screws and from the screw holes into the main part of the carborator.

Compression test as follows (using starter crank cold since I haven't been able to get it running)

Cyl 1: 125psi
Cyl 2: 115psi
Cyl 3: 125psi
Cyl 4: 130psi

The insulation on many thin wires has disintegrated and I have relpaced it with electrical tape. I had to do a splice repair 2 two wires that had coorodade through last year. (since that was last year, it has run several times since the splice) I would love to replace the wires altogether, but they are attached to a plastic ring under the magnito that I cannot remove.

I am not a mechanic. I do have a lot of tools becuase at one time I thought I was a mechanic. I am doing this work myself becuase the only mechanic who agreed to work on it last year let it sit in his yard for a month and a half and never looked at it.

Any advice would be greatly appreciated. Please keep the language layman freindly.

 

[Chris1956]

Re: Completely confused by 1976 Mercury 850

First get a Merc Service manual. I would recommend you install a new Merc motor wiring harness. The problem you have with starting and acceleration (since you say you have spark), is likely dirty carbs. You will need gasket sets, floats and inlet needles to rebuiild the carbs.

 

[CharlieB]

Re: Completely confused by 1976 Mercury 850

Ditto everything Chris has said.

2 strokes need an overly rich idle mixture for crisp acceleration.

After complete cleaning and rebuilding your carbs, adjust initial idle mixture screws out 1 and 1/2 turns from lightly seated, this should be enough fuel to get started.

On the hose set idle mixture to Best idle PLUS almost 1/4 turn out (CCW) This should be enough to get started on the water and IN Forward gear. There final adjustment should be made to Best idle (again, IN the water and IN Forward gear) PLUS almost 1/8 turn out
(CCW)

Test rapid acceleration, ANY even slightest hesitation, lean cough, bog, proves a need for more idle mixture. Turn each screw out the additional screwdriver blade width. Repeat test and adjustment to perfection.

The additional load of pulling up a skier often requires more idle fuel than a lightly loaded boat, if you experience acceleration problems with a skier, recheck your idle mixture adjustment just a hair fatter.

 

[Naaox]

Re: Completely confused by 1976 Mercury 850

Hello, I've been reading forums here for well over a year now but am a new member... I have been having this exact same problem with my late 70's mercury 850 thunderbolt. However I have already had a mechanic install a new wiring harness, switch box, and stator. I also had them do a full carb rebuild and tune on both carbs, due to the fact that there was bad fuel sitting in the tank and motor for roughly 2 years before I received the boat. Oh they also adjusted the timing. And told me that it was running great. I took it out the following day and it ran great, instant acceleration.... for about 10 minutes I stopped to get a beverage but left the motor in idle. As soon as I got back on the throttle it began to bog and eventually die if I tryed to accelerate to fast. I am leaning towards replaceing the fuel lines, tank, and filter but wanted to get a second opinion. Thank you

 

[194269]

Re: Completely confused by 1976 Mercury 850

I would also suggest you change the fuel line hoses. They are not compatible with modern fuels and may be disintegrating and plugging up the carbs.

 

[Naaox]

Re: Completely confused by 1976 Mercury 850

I would also suggest you change the fuel line hoses. They are not compatible with modern fuels and may be disintegrating and plugging up the carbs.

Hello again.
Ok so I replaced the fuel line, bulb and the quick release adapter that connects the line to the motor. I am still having this same issue. I took it out yesterday and found that if I get it to the mid range rpm where I am having the problem about 1500-1700 and right as it starts to bog down press and hold the choke button for about 1-2 seconds it will rev up rpms and I am able to get up to speed faster. By my understanding that means I am running on the lean side and should adjust to a slightly more rich mixture. Can someone please verify this. Thanks

 

[Chris1956]

Re: Completely confused by 1976 Mercury 850

Sounds like your carbs got gummed up from the old fuel/old fuel lines. The best fix is to remove the carbs and clean them out again.

Do you have a good water seperator fuel filter? If not, that is a real good idea as well.

 

[194269]

Re: Completely confused by 1976 Mercury 850

And replace the fuel lines that are on the engine.

 

[sweet duck]

Re: Completely confused by 1976 Mercury 850

Just got done with the same problem on my 850. Clean carbs and change the fuel pump diaphram. I was having to choke the engine to get up to high speed and I was using a more than normal amount of fuel. I pulled the fuel pump and found a cracked diaphram. While I was at it I took the carbs off and cleaned them. Runs great now.

 

[Naaox]

Re: Completely confused by 1976 Mercury 850

I will check into the water separator fuel filter asap. Thanks for the great tips guys. Ill let you know what I find.

 

[MrMichael]

Re: Completely confused by 1976 Mercury 850

Thank you to all who posted.

I started by going to to my local parts supplier. I had cracked the fuel pump during the diagnostic tinkering, and needed prices for that and the carb kit. Locally, the fuel pump and 2 carb kits would run me $140. Not bad but he also informed me that parts were no longer being made for my motor, so if another problem comes up there is a good chance I wont be able to get parts. I had tried to replace the wiring harness last year and am unable to remove it. For the above reasons I started to question the wisdom of sinking more money into this motor. I also did a quick check of my banking acount and if the boat had been running, with fuel costs and pay cuts being what they are I could have afforded one day on the water so far this summer.

Again, thank you to all who helped. You helped me to better understand what I needed to invest and come to the conclusion that it is time to cut my losses.

 

[CharlieB]

Re: Completely confused by 1976 Mercury 850

Go here to get the original part #'s for everything you need, it will show what parts are still available or not. Write down ALL the #'s then start searching iBoats and Sierra as both have a surprising amount of parts that Merc lists as NLA.

http://www.mercruiserparts.com/mercruiser_parts.asp

Check this thread to see if your model service manual is still available.

http://forums.iboats.com/showthread.php?t=309079&p=2072211#post2072211