Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

[stracey777]

I am just getting back into boating. I have a '75 Mercury (serial 4300903) 850.

When I prime the engine, gas pours out the top carburetor. My first thought was the float was stuck. I put a new float and carb kit in. I tested the fuel valve for the float chamber by trying to blow into the fuel in while lightly applying pressure to the bottom of the value. It is working. So I was confused after putting the carbs back in, that I still had the same issue. So I took the top of the float chamber off and primed the gas. No gas was coming from the gas line, before gas was coming out the front of the carb. I then took off the fuel pump and noticed there is a chamber in which the gas flows through. Though I have a repair manual, it is one of those that cover about 15 years of motors, so it does not appear to have clear diagram of the path for the fuel pump in this engine.

My question is:

Is there any diaphragm or other part internal to the engine that needs to be addressed beyond the fuel pump itself? The boat has sat for about 4 years without any interaction. The engine will turn over. Before I continue to throw money at this issue, I'd like to know if I am on the right track.

Is there a better manual for this engine model?

My thought now is to buy a fuel pump kit, as it looks like one of the 'values' is missing, but I cannot find a good enough diagram to be sure.

 

[emckelvy]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

It does sound like your fuel pump has a ruptured diaphragm. A good clue is if the primer bulb never gets hard when you pump it up, and you're sure there are no problems in the carbs. Fuel will go thru the ruptured diaphragm and flood into the cylinders. And right out the carb, so it appears!

Here's an example of a complete kit which includes the check valves:

Mercury Quicksilver 21-30430a13 - Valve Kit - Check @ MarineEngine.com

And a less-expensive kit which just includes diaphragms, gasket, O-ring (as required):

Fuel Pump Kit - Sierra - iboats

Pay special attention to the pump gaskets, make sure the old ones match the pictures of the new pump gaskets. There are subtle differences in gaskets and if you get the wrong ones, you may get a vacuum leak, and who needs more trouble!!?

Here's a link to the full parts list for your motor:

Mercury Marine Model 850 85 HP(4 CYL.) Outboard Motor Parts by Component

Last thought on the fuel pump check valves, they are held in by a metal retainer & 2 screws, so it'd be pretty hard for one to fall out. Unless someone had messed with the pump in the past. And if one were indeed missing, the fuel pump wouldn't pump correctly. Note that one check valve will face inwards, and the other, outwards, as they are mounted on the pump. At any rate, the full repair kit has 2 new check valves, so that should solve that problem, along with the suspected bad diaphragm.

HTH..........ed

 

[stracey777]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

Thank you! My search only found the kit with out the values, it is clear now that one is completely missing. If this is all that is wrong, this will be a nice boat, but of course the acronym still holds..."B.reak O.ut A.nother T.housand"

Also, I noticed, that when I turn the engine by hand using an air hose on the engine side of the pump, the air flow would vary, so it goes right along with what you are saying.

Thanks again, I will order the kit and post an update after...hopefully that it is running!

 

[stracey777]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

Okay, so it turns out the diaphragm was missing completely, so the fuel ran straight through the engine out the top carb. I have bypassed all of the bad wiring in the harness, by hot wiring the engine. It will start, however, it runs at what seems like full throttle. Since the controls are all taken apart, I guessed at what lowest setting by turning the distributor to the opposite setting. So now I have "faster", and "I want to blow the engine!"

Since this engine hasn't run in three years, I want to start at an idle.

...(Update) The carb was not installed correctly, maxing out the both throttles. Unfortunately, I managed to loose spark again. I am sure it is a wire, or something else loose, since I had to remove the switchbox assembly to get to the carbs, and it worked prior to this.

Thanks!

 

[Chris1956]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

To be clear, if the fuel pump diaghram was missing, raw fuel was leaking into the top transfer port. It should not have been exiting thru the top carb, in any great amount. Maybe a drop or two would pass thru the reed valves.

I would consider this motor to have been worked on by morons, until proven wrong. I recommend a Mercury Service Manual. If this motor is new to you, I would recommend a new waterpump impeller, wearplates and gaskets, clean the carbs and install new floats, gaskets sets and inlet needle and seats. Do a link and synch and inspect all wiring.

 

[stracey777]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

Yes, it seems that it was clear to be morons. I have completed the Carb rebuilds, bypassed the on board fuel filter (cover cracked) for an external one. (The on board fuel cover was $36, and the screen looked like it was made to filter pin needles, and not smaller debris) I was hoping to get the motor running, but the general consensus is replace the water pump impeller, so that is next on the list. All other gaskets seem to be okay, I am not in a hurry to strip the engine down.


The manual I have covers MANY models, it is a little tricky to read, but it is getting the job done.
(Online https://recrepairinfo.com/login.php )

If anyone knows of a better manual, I'd like it

ENGINE FINDER
The following listings contain all engines covered in this manual
Model/Engine Year
45 hp, 4 cyl ...................... 1965 - 1970
50 hp, 3 cyl ...................... 1986 - 1989
500/50 hp, 4 cyl ................... 1965 - 1985
60 hp, 3 cyl ...................... 1984 - 1989
650/65 hp, 3 cyl ................... 1972 - 1976
650/65 hp, 4 cyl ................... 1965 - 1971
650,700/70 hp, 3 cyl ............... 1976 - 1983
70 hp, 3 cyl ...................... 1986 - 1989
75 hp, 4 cyl ...................... 1984 - 1986
80 hp, 3 cyl ...................... 1987 - 1989
800/80 hp, 4 cyl ................... 1969 - 1983
850/85 hp, 4 cyl ................... 1973 - 1977
90 hp, 3 cyl ...................... 1987 - 1989
850XS/90 hp, 4 cyl ...................... 1976
100 hp, 4 cyl ..................... 1988 - 1989
115 hp, 4 cyl ..................... 1988 - 1989

 

[Chris1956]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

The gaskets and wearplate are part of the waterpump repair. I didn't recommend any tear down of the motor.

 

[stracey777]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

:embarassed: At this point, it really doesn't seem far off. How is it that one plug wire is SO expensive! Even the distributor cap is running $250! That's almost 1/2 what I paid for the whole boat!!

 

[Chris1956]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

Try the Oldmercs website or vintagemercs website for the plug wires. You will get them with the screw-on terminals, but will need to install the plug boots and springs yourself.

 

[dwco5051]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

The factory manual for this motor is C-90-75512. E-Bay usually has a few used ones on sale.

The cracked fuel pump cover is common on these motors. People would just keep tightening the screw rather than replace the O-ring when they had a leak. I always carried and extra cover and O-ring in my tackle box when I had my '76 850.

 

[GA_Boater]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

O'Reilly carries 7mm copper core plug wire in bulk. Same as Packard 440, which is original. I was able to get the wire at O'Reilly instead of waiting. Oh - $0.69 per foot. I splurged and got 10 feet. Re-used my plug caps because the O'R I went do didn't have them. BWD MW11 - Plug Wire | O'Reilly Auto Parts

EDIT - I don't know if the screw in ends at the cap end are re-usable.

 

[wired247]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

You need to do some shopping around. distributor caps , although rarely NEEDED , are under $100.

Lack of fuel pressure wouldnt force fuel out the top carb. Squeezing the bulb too tight will though.

 

[stracey777]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

The distributor is in good condition, however the connection of the wires is in question. I have decided they 'should' work. Though, the proof that a moron did the work continues to haunt me. As I started to take apart the lower unit, (going to check/replace the water pump) I have found there is no trim tab. Fortunately these are relatively cheap.

So far summary:

5 Kittens were behind the gas tank (Found them the day after I got the boat home)
Missing fuel pump diaphragm (Caused gas to pour directly out of the top carb)
Missing trim tab
loose wire in the circuit somewhere (No Spark, where there used to be)
Carbs are ready to go, the engine did start, but because I had installed the carbs wrong, the throttle was wide open, causing the engine to race, and after sitting for three years, That couldn't be good.

All things considered, not too bad yet. I will finish with the lower unit inspection and see what it needs. There is a big chunk missing off the bottom of the lower unit. Looks like it hit the ground on the road... Thinking about it, that may have been my fault... opps.

 

[wired247]

Re: Mercury 850 (85hp) - Gas pumps through the top carb - Failed fuel pump?

Trim tabs don't do much and some motor didnt have them at all.


5 live kittens?