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1980 Mercury 20 Hp, Fuel Issue

C

christianfreiss

Member
Guys,

I’ve got a weird one. The Merc is a 2 cylinder, with single carb. No issues there. It’s got good compression (120 on both cylinders). It has good spark (steady, bright, blue spark on both cylinders.

The problem...
When I start it, it is gutless. When I pull the plug wire to the lower cylinder, rpm does not change. I put the plug wire back and pull the number 1 plug wire. It instantly dies.

How does this happen? Could this be a reed issue or a crankshaft seal issue?

Thank you in advance for any assistance. I’ve been at this a while without success.
 
What does the cowling insulation look like? Been mice? Is the insulation breaking down? Why, you ask? Maybe stuff stuck in reed.
 
I’d be happy to pull it apart and examine the reeds if it is a possible solution. I guess my question is this. Could a damaged reed affect only one cylinder? If this is true it would explain a gutless Mercury 65 hp I’ve got as well.
 
Update:
I took off the carb and glanced in the case. I see 4 stainless reed/petals. They look great to me. Smooth. No cracks, bends, fully seated, etc. I did the listen test (put a hose down the hole, have a listen while someone else pulls the motor over with plugs removed). I heard what seems to be rhythmic suctions, no “breathing”. I don’t know if it was once per revolution or not.

Any more ideas?
 
Does spark jump a gap of 3/8" ( 1 cm ) on both leads , yes , no or not sure why to do that simple test.-----Post compression test numbers as well.-----Your 20 HP motor has 2 separate crankcases that are sealed from each other.-------Your 65 HP if a 1972 or newer has 3 separate crankcases.-----Yes reeds do operate for one cylinder only.
 
I'm with Jeff on this one.

If you test the spark and get good results on the lower plug lead then I would bet on a lower crank seal.

Fairly common issue on the early 80 something portables and a bit of a booger to change.
 
Is there a way to test the crank seal without taking it apart? Usually they will fail over time allowing adjacent cylinder to run lean. What are the circumstances that led to this failure of #2 cylinder?
 
Gentlemen,

I picked up a spark gap tester on the way home. I gapped it to 3/8 and checked both cylinders. It is definitely firing on both cylinders.

A little backstory:
I picked up the outboard a few years ago. It seemed to have good compression and the lower unit didn’t look bent, so I figured I’d fix whatever needed fixing.

I tuned it up (new plugs, impeller, oil grease, etc) then tried it in the water. It has run like a 1 legged pony since then. There was one time where when I opened it up it it faltered then took off like a bullet. I shot across the lake. After I let off and tried it again, no good.

I figured too that it was intermittent or weak spark so I replaced both coils, new cdi, new trigger coil.

Do you need me to get exact compression numbers?

I ordered a lower crank seal today. When it arrives, I’ll install it. In the mean time, does anyone have any other ideas?
 
Doesn't sound like a crank seal to me with those symptoms. Compression numbers are always critical....see what you can do. Stupid question. Did you try a different spark plug in effected cylinder?
 
Yeah. One of the things I did was rotate coils and plugs and wires “left to right”. It did not follow. Same root issue; when she is running, pull the plug wire on #2, no change to rpm. Pull plug wire to #1 (with #2 reconnected), does instantly.

I’m absolutely chasing my tail on this. Anything else I can try?
 
Gotta see poor compression number in cyl.#2. Did you say that the reeds for lower cylinder were fine? I dismantled a 1980 Evinrude 35 yesterday and lower cylinder was scored. There was crap sucked in a reed. Should have got a photo.
 
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Ok,

I did the “secondary” compression test. I’m seeing 130 on the top cylinder and 128 on the lower.

My next stop was the “primary” compression test. I had to do a fair bit of disassembly to get access to the lower crankshaft seal. I plugged the intake and exhaust and added about 10 psi of air to the lower cylinder. It was leaking heavily from the lower crankshaft seal. I checked for leaks anywhere else and noted that it was coming past the reeds pretty good. Visually they looked good, you could even reach in with your finger to unseat them and release to reseat them, but they just wouldn’t seat all of the way. I confirmed that this is problematic by testing the upper cylinder. Upper crankshaft seal and reeds are quiet.

I’ll get the parts ordered, installed, then reply with results.
Thanks again for the differential diagnosis. I wish I wouldn’t have replaced all those electrical parts. I have to remind myself there is a price for knowledge. This motor isn’t worth the $800 I have into it, but o’well.
 
So how can we explain the symptoms demonstrated in your post #10???? I agree the seal is bad, but post #10 led me astray. This is more common here in MN which is an ethanol mandated state. That means at least 10 percent of all fuel consumed must be ethanol. Mostly available in 10 percent form. How much you want for those electrical components? Got Mercs around here with no spark.....common problem. Just bought a 7.5, no spark.
 
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TimGuy,

I shouldn’t have given the back story. It confused things as it sometimes does with troubleshooting. It’s always best to tackle the central present issue. I’m used to wrenching, but not on outboards. Two stroke motors are still pretty new to me. I’ve never had a crankshaft seal fail, so I didn’t know really how to check for it or that it is a fairly common fail item.

I pulled off the case half, and sure enough, the inner ring of the seal was torn most of the way off. One of the reeds appears weak, but otherwise undamaged. It comes to rest just slightly unseated. I couldn’t see it until I disassembled the case.

As to the parts, I already canned the old parts, but thank you for the offer. I just learned a valuable lesson; make sure you do a primary (case) compression test on problematic 2 stroke motors. That would have drove my troubleshooting in the right direction earlier in the project. Thanks again for the support.
 
A bad crank seal won't effect the compression numbers. Only effects the fuel/oil/air ratio getting into the cylinder, so a compression test or even a leak down gives you no information about the upper/lower seals. Ditto with the reeds - same effect (jigger gas/oil/air ratio), but likewise doesn't effect compression.
 
A traditional leak down is done by connecting a leak down tester to a cylinder, adding a couple of psi, rotating the crankshaft until you find the compression stroke, continue to rotate it till your at tdc on that cylinder (usually you have to rock back and forth until you get it right), then you bring up the pressure. In typical 4 strokes your looking for 60/80.

What I did was plugged the intake and exhaust, applied pressure to a given cylinder, let it uncover the intake and exhaust ports (in theory it was at the bottom of its stroke), bring it up to about 8 psi (I read somewhere 6-17 psi), then listen-feel for air leaks. This I guess is referred to as a primary compression test. I’d never heard of it before this project, but it worked for me.
 
Okay. Not impossible then with that kind of a seal tear, it could have come into position for awhile and reseal good enough to draw fuel mixture. Remember that operation on a 2 stroke with a bad crank seal will lean and score adjacent cylinder/piston......as well as drying out bearings. This can happen in a fairly short amount of time.
 
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