Home › Forum › Ask A Member › 1941 Merc KB-3 Point Setting Vs. Coil Swap
- This topic has 9 replies, 5 voices, and was last updated 1 year, 3 months ago by Buccaneer.
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January 31, 2023 at 12:55 pm #272141
The original specs for the subject motor says point gap should be .018.
I did an OMC coil swap and set the points at .020, thinking that’s where
all good OMC’s have their points set.It appears to have good spark, but now I got to wondering.
Not that I think it’s the problem that’s keeping my motor from
running longer than 15 seconds, but does the optimum point setting
have more to do with the coil installed, or the laminates
and magnets position in the flywheel?I checked the compression this morning, and it has 87 psi.
Hopefully that’s enough for these little single cylinder Mercs.
Crank bearings were tight, new crank seal installed.
I sealed up the poppet valve cover and flange gasket better,
tried a new spark plug, etc., but 15 seconds or so is my longest
run, and that’s “out of water”.Prepare to be boarded!
January 31, 2023 at 1:27 pm #272143Well Mister Buccaneer here is my two cents on this question. Back when I was racing snowmobiles we ran the points a little wide. Wider points make more advance. Narrower points retard timing. Those old motors didn’t care much. Can you put a dial indicator in the spark plug hole and check where the points break? Your new coil might be sparking at a different spot than the original one. At start up you need about .020 thou BTDC to get you going. A dial indicator and a continuity light should tell you where the points break.
dale
- This reply was modified 1 year, 3 months ago by labrador-guy.
January 31, 2023 at 2:53 pm #272144I’m not an expert by any means, but I agree with Dale that the coils might spark at different times.
I would look at the magnet setup in the flywheel and see how it compares to an omc motor that uses those coils.January 31, 2023 at 5:19 pm #272152I would set them at .018. The spark happens when the points break, no matter what coil it has in it. Right? The cam is designed in conjunction with the points, the lamination and crank. I can’t see why you would want to mess with the point spec, just for a coil. The condenser is another story and I will let you guys flog it out on that. By now, everybody here knows MY feeling on condensers.
Long live American manufacturing!
January 31, 2023 at 6:41 pm #272154Well Mister Buccaneer here is my two cents on this question. Back when I was racing snowmobiles we ran the points a little wide. Wider points make more advance. Narrower points retard timing. Those old motors didn’t care much. Can you put a dial indicator in the spark plug hole and check where the points break? Your new coil might be sparking at a different spot than the original one. At start up you need about .020 thou BTDC to get you going. A dial indicator and a continuity light should tell you where the points break.
dale
- This reply was modified 1 year, 3 months ago by labrador-guy.
Advance / Retard on outboard magnetos is determined by where you
move the mag plate, hence I can make the points open “sooner” or “later”
by moving the plate.I can’t remember if the old snowmobiles had any kind of mechanical
advance, but the few times I did mess with them, from what I remember,
the specs were for the points to open a certain “piston travel” BTDC.I can certainly try setting the point’s to .018 before I designate this
little Merc to “Saw horse Queen” status.I messed around with any potential “crankcase” pressure losses,
checking bolts as “tight”, and had the poppet valve carb off twice,
the last time lapping the carb flange that mates to the crankcase.
I lapped the mating flange on the crankcase when I had it all apart.
I also messed around with different spark plugs.I did get it running a little longer, but it just won’t take the leap
of faith to stay running.
I checked the compression, and it’s 87 psi. I had lightly hone the cylinder
and unstuck and decarboned the ring grooves.
Out of desperation and curiosity, I checked the compression on
the little Merc K3-1A I was just given, and it’s 88 psi.
Unless they’re both “worn out”, one would think that’s enough!Video of today’s progress. I made improvements in length of run,
opening the needle valve slowly, up to about one turn open,
but from there, it was “down hill”.
Any ideas?
https://youtu.be/MF-LqCNsGj8Prepare to be boarded!
January 31, 2023 at 7:11 pm #272156I could tell you lots about timing. The old German/Austrian motors all had centrifugal advances inside the flywheel. Set points at .015 when the motor started it went to full advance. The Japanese motors came along and we set the timing by point gap that was built in too somewhere around .060thousands BTDC. CDI motors had no way to change timing.
Is it possible that the points cam might be upside down? You have plenty of compression. The motors usually still run with crank seal problems. Not good but they will still run. Maybe your poppet valve spring is to stiff? You have fuel coming? It’s gotta run!
dale
January 31, 2023 at 9:13 pm #272163Dale, since I bought my 1987 SkiDoo, I haven’t worried about timing, nor on
my 2000 Skidoo that I currently run…… just pull the cord and “Go”.
The Merc points cam has the little direction arrow “up”, unless something
happened inadvertently, but that’s a good idea to check when I pull the flywheel.
One can hear the poppet valve working good when pulling over the motor.
The crankcase opening for the carb to bolt on is “huge”, so lots of potential
there for loss of crankcase pressure, but I feel I have it sealed up pretty good
now. Forgot to mention before, but I also “richened up the fuel mixture,
thinking it would seal up the crankcase better, and perhaps “up” the compression.Prepare to be boarded!
February 1, 2023 at 5:45 am #272174Interesting. I guess if it were me, I would pull a known-good conventional carb and reed plate off one of my KD3s, and see if it would stay running on that. That way, you’d know whether it was the poppet valve rig or the crank case. I have a poppet valve, Montgomery Ward/Merc, that is very similar in design. It says running okay but it has a weird behavior. It will be running strong, then drop to half speed, then come back strong, just EXACTLY as if it were a two-cylinder engine, dropping one cylinder in and out, like turning a switch on and off. I can’t imagine what does it; but it’s the only one-cylinder, Merc-based engine I ever had that did that, and it’s my only poppet valve Merc.. The point is, I don’t fully understand or trust the run-ability of those simple valves. My only other thoughts are, that you don’t have a ton of spark advance there…I know it’s not on a boat but I would still try a little more advance. Also, where is all the smoke going??? I see none in the video. Those things have to have 16:1, in my opinion.
Long live American manufacturing!
February 1, 2023 at 7:58 am #272175I like Bills plan.
always go with a known.
February 1, 2023 at 9:21 am #272178Interesting. I guess if it were me, I would pull a known-good conventional carb and reed plate off one of my KD3s, and see if it would stay running on that. That way, you’d know whether it was the poppet valve rig or the crank case. I have a poppet valve, Montgomery Ward/Merc, that is very similar in design. It says running okay but it has a weird behavior. It will be running strong, then drop to half speed, then come back strong, just EXACTLY as if it were a two-cylinder engine, dropping one cylinder in and out, like turning a switch on and off. I can’t imagine what does it; but it’s the only one-cylinder, Merc-based engine I ever had that did that, and it’s my only poppet valve Merc.. The point is, I don’t fully understand or trust the run-ability of those simple valves. My only other thoughts are, that you don’t have a ton of spark advance there…I know it’s not on a boat but I would still try a little more advance. Also, where is all the smoke going??? I see none in the video. Those things have to have 16:1, in my opinion.
I have the KB-3 ‘s cousin on the workbench now, and it’s
a KB-1A, “no thrills” version, that has the reed plate
and conventional Tillotson AJ carb on it.I’m not sure though if the reed plate will go on the KB-3.
The garage is warming up now, but I’ll find out soon.I tried moving the mag plate all over, and about 2/3 rd’s
advanced seem to run it best.It had 16:1 fuel mix in the tank, and I gave it a good dose
of 8:1 to richen it up. I wondered about the “lack of smoke”
myself…….. I usually have to evacuate the garage after 30
seconds! Maybe the 20+ below weather has depleted
the oxygen level!Prepare to be boarded!
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