Mercury KF-7 lightning – bottom cylinder drops out.

[Tubs]

Got the motor in 2016. That winter the motor
got new coils and condensers, (coils were still
available for the Bendix at that time) upper and
lower crankshaft seals, gear case seals, and some
gaskets. The power head was not disassembled. I
believe compression was around 120 in both
cylinders. It got used a bunch through 17,18 & 19.
Started easy and ran great. It was fine at the
beginning of 2020 but after about month it started
missing. We couldn’t find any issues with the
ignition so we suspected it was the mag itself that
was failing. Got a Phelon magneto to replace the
Bendix. Converted it to the OMC style coil.
Everything new except the points. Tried running it
the last couple of weeks. Same thing. 3 new sets of
plugs and numerous carb inspections, including a
new float, have not resolved the issue. It appears to
be the bottom cylinder that is missing. It starts right
up and fires both most of the time at full throttle
but one starts to drop out as you reduce RPM.
Every part in both magnetos were replaced with new
parts if available, including plug wires and their ends.
Everything is different, including the brand, yet it
runs the same. A Merc-O-Tronic doesn’t reveal any
issues with either mag. We are clueless as to where
to look next.

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

• This topic was modified 4 years, 4 months ago by Tubs.

[dave-bernard]

try a ground wire from the mag to block.

[billw]

My two thoughts would be either the bottom cylinder is ingesting a bit of water from a bad lower crank seal or inner exhaust plate, or maybe if it has had a new lower crank seal, one was installed without the tiny drain hole in the seal’s case, as was often seen in that vintage of 20 cube Mercury? That would mean the bottom cylinder couldn’t get rid of its puddled fuel correctly. Dave’s ground wire idea is sure an easy test, though, before you get any deeper.

Long live American manufacturing!

[Tubs]

Thanks for responding.
If it is water causing the misfire should we
expect some indication of that on the spark
plug? I put the seals in the motor in the
winter of 16-17. I don’t remember if there
was a hole in the bottom seal. I may have a
picture of the seal I put in. I’ll check to see
if I can find it. What would be the best way
to check to see if it is oil collecting in the
bottom cylinder?

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

• This reply was modified 4 years, 4 months ago by Tubs.

[billw]

I guess if the cylinder was completely non-functional, you’d see some water. Sometimes you have to tap the plug on your oily palm and watch for beads of water. Take the plug out right after running the motor when the problem happened. As for the seal and the hole, good question. Best way is to take it apart and look; but who wants to do that on a whim? What if you just pulled only the power head and sprayed some carb cleaner down the carb, and turned it over a few times to see if residue came out of the crank area? The last seal I saw with a hole, had a tiny hole….like about a #52 drill bit. That was in lieu of any formal crank case drain system.

Now that I re-read your post, it ran for three years okay; so forget about the hole in the seal. what if your lower seal has failed? You’d loose the crank case pressure at lower speeds

Long live American manufacturing!

[Tubs]

I found the picture of the seals. We took out the
plugs right after running it and didn’t see any
evidence of water on them. Something with
the seal is the only thing I could think of but not
having much experience with these I’m just
guessing. I appreciate your advice. Thanks so
much. It makes me feel that is the direction we
should be heading. I’m wondering if maybe
it has worked, or is working, its way out? I gave
this motor to a friend when I went back home in
2017. I just bought another this spring. A winter
Project to bring next year. They both need to be
running. We’re both geriatric delinquents. (He’s
68 and I’m 75) It’s going to be a lot of fun for us.
Our wives will just be embarrassed.

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

[billw]

Well, there’s the hole in the old seal….That’s the crank case drain. My thought is maybe you got a new seal that wasn’t really designed to tolerate gasoline and it failed prematurely? There is a way to test newer Mercurys for crank case pressure but it relies on already-installed, small nipples, that you attach to a small check valve and a pressure gauge. The check valve cancels the negative pressure pulse and allows the gauge to just read the positive pressure. Obviously, KG7s have no such nipples.

I usually coat my gaskets with oil before I install them. That way, I have a chance of taking them apart and maybe be able to reuse them one time. Too bad you couldn’t just loosen the cylinder block from the crank case and pull the lower bearing carrier off and have a look at the seal.

Long live American manufacturing!

[Tubs]

The seals came from Joe Poole. We need
to check that seal so it’s coming apart. If
its not the problem at least its off the list
of possibility’s.

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

[Tubs]

Took it apart last night. Seal is rock hard with
a small piece missing from the edge. Put the
lower bearing carrier in the freezer, We’ll See
if the bearing will drop out later today.
Thanks again.

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

[labrador-guy]

TBH, if that seal has a chunk out of it, that little hole that is in the seal don’t mean beans! The lower cylinder probably wasn’t getting the proper amount of fuel. I hate to dump extra gas/oil mix in the water unnecessarily. JMHO

dale

You’re never to old to learn something stupid!

[Author]

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