low tension magneto question

[johnyrude200]

that’s what the low-tension magneto uses. CD uses blue orange/orange.

[fleetwin]

Guess I’m confused….This engine ran at some point, correct? The timing is not out 180 degrees if the engine ran. What changed on the engine between the time it ran and when you felt the timing was out 180 degrees. Was the engine running when you felt it was 180 degrees out?

[johnyrude200]

No no, I was installing the low tension magneto and timing was off 180. It never got to the point of running; this motor was built esentially from scratch.

I couldnt figure out what I was missing with the LTM so opted to switch to CD and it works fine. HOWEVER, I still need to know what I missed for the next one of these I deal with, which is fairly frequent.

My post from a couple of knotches back explains where I stopped fooling with it.

[fleetwin]

yep, am confused again, can’t keep up will all your projects!

[rudderless]

I have been working on these systems for years. Back in 1970 or so most snowmobiles had this system.

There is a thing called the right hand rule. Make a thumbs up sign with your right hand. The thumb points in the direction of DC current flow in a wire, and your fingers point in the direction of the magnetic flux. Point your thumb down and the flux travels in the other direction. Keep this in mind.

The driver coil is not center tapped or grounded on either end. The flywheel is NS then SN so the current can reverse direction in the driver coil for each cylinder. Why so one might ask. The spark should jump from the center electode to the ground. In the 9.9/15 configuration if wheel was NS,NS the spark would jump from the ground to the center on one cylinder, causing fast erosion of the ground electrode. Right hand rule rules.

Okay, points close on one end of the driver while cap and points are in parallel with the coil on the other end of the driver. The points on the cap end are open, not in the circuit at this time. Magnets cause current flow, energizing the driver coil, spark coil and cap. Points open at highest mag flux causing the flux to collapse into the secondary winding in the spark coil, causing the spark. The degausing cap keeps energy in coil from bleeding out through the point spark as they open. Without the cap the energy would follow out with the point arc, causing weak spark or none at all. The cap discharges keeping the potential about even on both sides of the points as the points open. When the cap has discharged the flux stored in the iron is kept strong and collapses into the secondary winding causing the spark.

Things switch for the other cylinder. Points that were open during other cycle are now closed and other points are now open, out of the circuit.The other magnets are switched causing current to flow in the other direction, towards the other coil.

The fixture you have times the magnets with the point opening for strong mag flux. There is no way for you to have it 180 out. My 1976 factory service manual explains this pretty well. Not sure if the other years have this explanation. Don’t go by wire colors on used stuff, could have been screwed with in the past.

There is my first post on this forum..hope it went well.

[fleetwin]

Sounds good to me!

[frankr]

OK johnnyrude, after much study (again), I think I have the problem figured out. I have been making those timing fixtures for over 3 years, and nobody has complained. Now you come along and say it is 180 degrees out. You may be right.

Picture from the service manual shows #2 points to be on the left (that would be #1 if it were the Universal Magneto).

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Photo from manual rotated to put front at top of picture. Note striped wire on left points

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Wiring diagram in manual has front of motor at right side instead of top of page. Infers that the upper coil is #1 (WRONG)

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Rotating wiring diagram to put front at top changes position of coils. Upper one is #1 (correct).

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So, what’s wrong with the timing fixture? They changed the position of #1 and #2 points (swapped left to right). So, timing fixtures are stamped with T & B ends swapped to match. HOWEVER, I now see that OMC milled the crankshaft key slot 180 degrees from the old position, which puts T & B back where they used to be.

johnnyrude, I am going to sent you a corrected timing fixture. Please let me know how it works out
Frank

[johnyrude200]

Well, I’m glad to see I’m not crazy. Frank I looked at these same diagrams very carefully prior to putting anything up in the forums a few days ago and glad to see I may, possibly, not be as dumb as I look.

As I mentioned in my earlier post, I could easily have reversed the magneto leads out to the coils when I had a 1/2" spark flowing from both coils, but on principle, refused to do this as a ‘shortcut’ to protect my reputation, should another mechanic ever service a motor I have laid hands on. As the saying goes ‘poop floats.’

I guess the other ‘take home message’ here is that either this system works correctly, or not at all. At least in my limited time working on these (say the last couple of years, maybe 40-50 units), they either throw 1/2-3/4" spark, or they barely show anything at all (say 1/32" of weak output). And that is assuming points, coils, condensers, and charge coil, along with all wires and connections with good grounds are working properly.

Your sherlock holmes detective work also reveals that BLUE/WHITE is in fact #2, not #1 in this instance.

Again to Fleetwin’s and T2Stroke’s advice…"Stay away from the low-tension magnetos!"

[frankr]

I just finished making a reversed timing fixture, but I’m more convinced than ever that OMC didn’t make those things all alike. According to the junker motor that I have, I’ve been making the timing fixtures correctly all along. Right now, I’m not trusting the motor OR the book. But one thing I’m sure of, on the 9.9 powerhead that I have, the crankshaft keyway is at 9 o’clock at TDC. And the Universal mag motor that I just tore down to look at, the key is at 3 o’clock at TDC.

I dunno. BTW, PM me your address so I can send this thing out to for inspection.
Frank

[johnyrude200]

The one thing that had me turned around is using the timing fixture with "T" and "B," this did not necessary correspond to when the dimple on the timing cam began opening the respective point (or so I surmised). On the universal mag, if you are looking at the mag plate from the AFT/REAR of the motor, top is on the port side, bottom starboard. On the low-tension mag, it is reversed based on what we’re talking about. Of course if we consider the prospect of the crank keyway also being reversed, now we have a head-scratcher situation (on top of the diagrams being a bit ambiguous to the untrained eye).

I think the diagrams leave a lot to be desired in the service manual, and your extra notes posted on the pics should be something that makes this post a ‘favorite’ somewhere on these forums so that future readers can understand this scenario.

The 1st two ‘theory of operation’ pics you put up, notice you can’t tell where the striped/solid point lead wires go to. The black lines representing wires out to the coils aren’t labeled. So when you progress to the wiring diagrams at the back of the manual which DO show what wires go where, it still doesn’t let you figure out how to set the timing using a fixture based on where the crank is turned because those wiring diagrams don’t show the corresponding position of the timing cam. (!!!!!!!!!!!!)

The 2nd item is that the wire color/stripe leads from the charge coil/points is very difficult to see in these older service books. The large images you posted are easier, but the real-life pics you pulled these from are only about 2" x 2" in dimensions and use of a magnifying glass is a must in reality.
And I have 20-10 vision and am 37…so forget it for the untrained/foggy eye.

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