No Fuel To #4 Cylinder Merc 850

ddtacm · Dec 23, 2012

Hello ,

I am still working on the restoration of my 1977 Mercury 85 HP. New fuel pump, rebuilt carbs, new spark plug wires, I have new plugs, nice blue spark on all 4 cylinders , I noticed after trying to set the Idle it just wont Idle correctly.
It is 135 PSI on all 4 cylinders. After checking everything again i removed and inspected the plugs. The #4 plug still looks brand new. The other 3 all look identical(after exstensive idle) I replaced the #4 and checked for spark before installation and have nice blue spark. I then started the engine and ran the same (poor Idle) I then disconnected the #4 spark boot and put it on a spare plug and have great spark. and the engine runs the same when it is connected to #4 or disconnected. It is not getting any fuel. Can anybody tell me why this may be happening and how to properly step through the repair proceedure to correct the problem. Thanks.

boobie · Dec 23, 2012

Re: No Fuel To #4 Cylinder Merc 850

Get some premix fuel in a spray bottle and spray some fuel in the carb that feeds # 4 cyl. If it picks up you still have some carb probs.

ddtacm · Dec 23, 2012

Re: No Fuel To #4 Cylinder Merc 850

The motor has 2 carbs. both are clean and main jets clear, floats in both are adjusted correct. I will try the spray bottle feed and see if it picks up.

Moody Blue · Dec 23, 2012

Re: No Fuel To #4 Cylinder Merc 850

Chances are that you ARE getting fuel into #4 cyl if the #3 cyl is getting fuel. The lower carb feeds both #3 and #4 cyls. You are probably ALSO getting water into the #4 cyl resulting in the steam-cleaned appearance of the plug.

Pull the #4 plug and using a bright light, have a look into the cyl. Bet you that the piston top is also nice and clean and shiney.

Water intrusion will also result in a poor idle operation because the fuel mixture is being diluted with water vapour.

Pretty common on the older in-line motors. I'm doing my 1971 Merc 800 this winter.

Faztbullet · Dec 23, 2012

Re: No Fuel To #4 Cylinder Merc 850

Likely bottom seals are bad.....

ddtacm · Dec 23, 2012

Re: No Fuel To #4 Cylinder Merc 850

What is the next step? Pull the powerhead for inspection and install new gaskets and seals ?

Maxz695 · Dec 24, 2012

Re: No Fuel To #4 Cylinder Merc 850

If you remove the LU you will usually see white ish crap indicating a blown crankshaft seal. Take a flashlight and look up at the bottom of the crankshaft for this leaking. If none is appearent. I would remove the exhaust cover and baffle on the side of the block and inspect it for a bad gasket corrosion and also look through the exhaust port and inspect the piston for corrosion and ring play to be sure there springy and sealing. With 135 Compression it should be ok. Replace any cracked plates and install new gaskets. Pay particular attention to the lower portions of the exhaust port and plates as this is normally the cylinder that takes the hit when intrusion occurs.

ddtacm · Dec 24, 2012

Re: No Fuel To #4 Cylinder Merc 850

Thank You Max,

As a matter of fact when I brought this boat home I noticed the bottom bolt on the exhaust cover un-screwed I screwed the bolt back in, it was not easy due to lack of access but I was able to get it back in. Do you feel that is where my problem lies? Do I need to pull the powerhead to fully access the exhaust cover plate or is there an easier proceedure? Again thank you for sheding light on this problem. The motor is very clean and in great condition for its age, most definetly worth digging in to to resolve any issues. When I installed the new water pump I had a few issues with the shift spline. But while inspecting the crankshaft area with a high power flashlight it looked like new in there.I guess my question is : Can the exhaust cover gasket be pulled and replaced without pulling the powerhead? And is there any tools you would recommend for the job? I have restored trucks and motorcycles in my day but not a lot of knowlege on boat motors, only bleesed with the mechanical ability to do it.....

Maxz695 · Dec 24, 2012

Re: No Fuel To #4 Cylinder Merc 850

View attachment 177507 (My 70 HP cover removed )Although I,m not firmiliar with your block If you can remove the throttle arm and shifter bracket there may be enough room to remove the cover and baffle without having to remove the powerhead. Be careful not to break the bolts as some times they can be stuborn and a little back and forth movement at a time usually loosens up the corrosion. Take your time in doing this and it shouldn,t be a problem. Pictures of my cover and linkages View attachment 177509 View attachment 177510

mercurymang · Dec 24, 2012

Re: No Fuel To #4 Cylinder Merc 850

I just went through this process on my '75 650 and ended up breaking three bolts. I probably could have drilled them myself but I opted for pulling the power head and letting the machine shop do it. Only $70. I replaced the two lower seals and o-ring. I also replaced the baffle plate, outer cover and everything inside there like the poppet valve and thermostat, etc.

ddtacm · Dec 24, 2012

Re: No Fuel To #4 Cylinder Merc 850

I just got finished pulling the exhaust cover off my 1977 85 and everything looks good i have attached a couple of pictures also.
Checked compression befor pulling cover and it is 140 cold. Skirts and rings look new on all 4 cylinders with just a tiny bit if skirt wear on #3 premix running out of #4 exhaust outlet(shown in pic) witch tells me no problem with fuel. Is it possible that #4 coil is not popping under pressure. It has nice blue spark when checked grounding spark plug to housing. I also noticed the #4 coil has been changed by previous owner, possibly a problem in the past. I am hitting a wall here, everything I check looks too good.Any advise will be appriciated.
By The Way, Wishing everyone that reads this a Merry Christmas ! And May God Bless You And Yours !!

Maxz695 · Dec 24, 2012

Re: No Fuel To #4 Cylinder Merc 850

It appears that is alot of fuel but thats more than likely because it,s unburnt. Compression is good you say. The pic for me is hard to see as to the right of the baffle in the picture appears to have fuel oil mixed with water. This would be in the second picture lower right upper quadrant.I don,t think it would be enough to stop #4 from firing but would cause it to run with a miss or fouled plug. Check the entire plate real good for hairline cracks both the baffle and the water cover. Replace the gaskets and reinstall them. I would try a new plug in #4 after this is done. If you have a problem aftert that I would check the firing oreder and make sure the plug wire isn,t arching or crossing another wire making it fire at the wrong time which would give spark but not at the correct time Merry Xmas I have included a revised picture for you to veiw in hopes it might be of some assistance Please click to veiw in a larger form thanksView attachment 177527 Water ports in this pic View attachment 177533

ddtacm · Dec 24, 2012

Re: No Fuel To #4 Cylinder Merc 850

I will check for hairline cracks. Firing order is correct and plug wires are new. (Installed yesterday) also new plugs. I will order the gaskets
and re-assemble exhaust cover after separating and checking for cracks.

ddtacm · Dec 24, 2012

Re: No Fuel To #4 Cylinder Merc 850

Also please note: The plugs that have been removed from #4 have all been as new condition just as they came from the box. This includes the plug that was in #4 when I received motor.

Maxz695 · Dec 25, 2012

Re: No Fuel To #4 Cylinder Merc 850

You might want to take off the coil for #4 and inspect it I had spark on all cylinders but one would cut out when throttling up . My replacement coils showed last night. I tested the old coil and plug wires with an ohm meter and saw one wasn,t getting current from the coil plug insert to the end of the pug wire. The core was broken. I had replaced one already that had a broken core but I missed this one. I just installed everything and rechecked them and now have continuity on all 3 coils to the plugs now. It fired up and sounded good. I,m gonna try to get it over to the ramp in a bit and give it a go.

ddtacm · Dec 25, 2012

Re: No Fuel To #4 Cylinder Merc 850

Thank you Max,
I am planning to re-check all wires even though I Just replaced the plug wires with OEM replacements. All check good. The old wires are still good also(still look new) Also the #4 cylinder is getting nice blue steady spark at idle( with a plug screwed in to #4 cylinder running on 3 cylinders)I am going to speak with the previous owner and get some more back round information on this outboard, everything just looks to new to be a 36 year old motor even though the serial is correct for a 1977 it looks like the whole motor was replaced at on time or they just didn't use it much. I do know it was only used in fresh water and kept under a parking carport. The #4 coil is a replacement coil so that tells me that someone checked this motor before me. The previous owner always paid a shop for any repair or maintenance
done to motor. I will be able to spend more time on it this weekend and hopefully will find out more back round service done to motor
and pull the wire diagram and double check all wires from the trigger plate to the switch box and then to the coils..... Thanks, Dan

Maxz695 · Dec 26, 2012

Re: No Fuel To #4 Cylinder Merc 850

My testing failed yesterday. Looks like i,m gonna be needing a trigger. The engine runs good then a cylinder cuts out when throttling up to about 3500 rpm. I replaced SB Stator coils wires plugs but still having the problem. It has to be a bad trigger coil.

ddtacm · Dec 26, 2012

Re: No Fuel To #4 Cylinder Merc 850

Sounds good, I have double checked all wiring on my motor and it is correct. I have ordered new gaskets for exhaust cover and going to clean and inspect with a magnifying glass. If all is good I will reassemble this weekend and continue troubleshooting my engine. I hope you have good luck getting your electrical problem shorted out. I wont give up till i find the problem with mine...

quicktach · Dec 26, 2012

Re: No Fuel To #4 Cylinder Merc 850

Check to see if the trigger coil or it's wiring is binding anywhere in it's travels. As well check all linkage moving the trigger assembly as well as a smoothly moving plate. Did you test the resistance of the trigger coil and did it check OK ?

ddtacm · Dec 26, 2012

Re: No Fuel To #4 Cylinder Merc 850

I have not checked the resistance of the trgger coil yet. I get off work late each day and ran out of daylight. I will be checking all electrical and coils next.

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