Mercury 100 ELPTO trouble --won't run on 4

Minnboater

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Jul 9, 2011
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Really need some help. My 1989 Mercury ELPTO refused to plane at the start of my annual spring walleye trip. Started fine but got only a strange popping sound at half throttle and above. Assumes this means the 2x2 system never kicked in and never jumped to 4 cylinders firing. I changed the plugs at the dock but got the same result. Ran into town and picked up a fuel pump rebuild kit from the local Merc dealer. New symptom: thought I heard an odd clicking noise at idle. With my fishing trip in tatters I ran the boat onto the water again but got no better result. The engine died after about 10 minutes. I'm worried my hasty repair interfered with oil getting to the carbs and I shelled one or more cylinders. Any thoughts? I'm going to borrow a compression tester tomorrow and see what I get. Would appreciate some perspective on my go-forward plan in the event the cylinders check out:

Step 1: confirm that fuel pump is assembled correctly.
#2: go through and clean all 4 carbs
#3 change the fuel filter
#4 punt

Couldn'd find anything in the service manual about how the 2x2 really works. From the diagrams of the carbs it appears like two of them lack idle jets. Does that mean if I've got two cylinders firing at idle that I don't need to chase an electrical problem.

I really qppreciate the help. My two sons missed a year of catching walleyes and are
hopeful we can salvage some water skiing. I've got a small amount of mechanical ability and not much money to spare and need to keep this a diy job. Thanks.
 

sschefer

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Re: Mercury 100 ELPTO trouble --won't run on 4

The way the 2+2 works is that below about 1700 rpms carbs 3&4 only supply enough fuel for lubrication. As you begin to accelerate an accelerator pump adds fuel to cylinders 3&4 via mechanical injectors located behind the electrical plate. There is a 1/8" supply line for the injectors that has an inline filter located behind the oil tank. Somewhere between 1500 and 2000 rpm, carbs 3&4 begin supplying adequate fuel for combustion and cylinders 3&4 begin to fire. The accelerator pump continues to add fuel all the way through to WOT so if the filter is plugged, and injector is plugged or the accelerator pump is bad, you'll have all the symptoms your speaking of. The ticking noise at idle could be the oil pump since there's really nothing else that can make a tick sound.
 

Minnboater

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Jul 9, 2011
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Re: Mercury 100 ELPTO trouble --won't run on 4

Thanks Steve, I'll follow your lead and see where it goes. I've heard the intermittant click of the oil pump before and this was something different. It was fast and rhythmic and I worried I was hearing pistons firing without lubrication. I had reservations about leaving the dock with that noise but pressed ahead anyway.
 

Minnboater

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Jul 9, 2011
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Re: Mercury 100 ELPTO trouble --won't run on 4

Progress report: All 4 cylinders have compression readings within 5% but I don't believe the 80 reading that my compression tester delivered. Just for giggles I tested my kicker which runs like a swiss watch and got a pair of 70s. I yanked off my rebuilt fuel pump and compared it with the schematic in my my service manual and everything looked good. Big learning for me:

Keep the service manual in the boat. I rebuilt the fuel pump at the dock last spring without a manual in hand. The pump chamber diaphragm has two big holes to facilitate oil entering the fuel stream. If you mix up the diaphragms you'll have straight gas going to the carbs. I was careful when I was swapping out parts but I think I dodged a bullet.

I'm going to focus my energies on the accelerator pump flow circuit. There's a small filter upstream of the injectors and check valves, so I'm going to rebuild the pump. Minnesota gas is full of enthanol and in 1989 Mercury probably didn't spec their parts for alcohol. I haven't pulled the pump yet but I went ahead and ordered a rebuild kit and hope to receive that next week sometime. The fuel and oil lines were intact but brittle. I'll change them out as well along with the two gas filters. I'll provide another update soon, hopefully saying that I've got an engine running on all 4 cylinders. Thanks again for the help Steve.
 

Minnboater

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Jul 9, 2011
Messages
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Re: Mercury 100 ELPTO trouble --won't run on 4

This progress report is long overdue. The accelerator pump I installed back in July took care of things and my big Merc was soon back to running on all 4.
 
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