Something I just thought of.

Using stevens meter you could snake a wire to the primary positive side of coil which is the one you have the red alligator clip to in first pic. Then place flywheel on motor where points have just broke for that cylinder and with stevens on spark check see if you get spark on both cylinders. Then rotate flywheel 180 degrees till points just break and see if you get spark on both plugs. That might also help detect if brushes are working correctly or if the ground brass plate in flywheel is indeed grounded correctly you should only get one spark on one plug wire.

Just thinking what I might try.

Joe

I’m sorry had to step out today also.

You wrote:
"Regarding your last idea, the flaw might be that in order to
get the coil to fire on the Stevens tester, or otherwise,
one side of the secondary must be grounded, therefore
I don’t see how you’d tell if both plugs fired at any given time.
Am I thinking wrong here?"

When the flywheel spins in real time it grounds one side of secondary automatically if brushes and flywheel tab and no cracks in bakelite and its all working.

That is why I was thinking you could put your flywheel in the position on engine and where coil would normally fire that should kill one cylinder fire if brushes and tab are working. Snake a wire and check with stevens meter.

By doing so your checking both plug wires with stevens meter to see if the other side of coil secondary is grounding out as it should with bakelite brass piece in flywheel . The brass tab did show a correct ground when you bench checked flywheel with omc coil and made sure tab was grounded and no cracks in bakelite. You did get spark with the probe when you got near brass tab but no spark checking bakelite thats a good thing. it means Good Ground on tab and good bakelite no cracks?? Hope that makes since.

One side of secondary should automatically be grounded when coil fires by flywheel brass tab and brushes are is working correctly which I suspect it isn’t. You have 2 lobes on cam so when one cylinder fires one shouldn’t because brushes. When cylinder fires one shouldn’t because other brush and by shouldn’t I mean they can’t or motor will never run right.

One more test to possibly try. Use stevens meter to fire your coil and see if both spark plug wires put out spark with flywheel off.
By checking one plug wire for spark then the other with flywheel off. I bet you a bunch of money that they both fire when one is not grounded and that will help explain the need for the brushes and how important they are for correct ignition results. What kills one side in real time is the brushes grounding one side or doesn’t kill the other side then you get double firing which I think you may have.

A zephyr has 4 cylinders 2 coils which means 1 coil has to fire 2 plugs/cylinders at same time but one is one piston is in the tdc position and one in the bottom dead center. Since their is 2 coils on a zephyr 4 cylinders more than likely 2 lobes they alternate all that and don’t need the bakelite contraptaction you have in your flywheel.

Check your capacitor for leakage also. Try a good tested condenser with good leakage reuslts. The motor does also sound like a capacitor that may be breaking down. With one side of coil grounded everytime it fires I would guess a good condenser/capacitor is even more critical. It really sounds to me like an ignition issue.

If you can’t get it worked out we might try one of my home made red atoms in it since you have a type of distributor using the flywheel and brass tab. I have only had success with the omc engines 2 cylinders 1950-1960ish but both magnets are on one-side of flywheel your magnets are on both sides of flywheel but with the flywheel used for distributor who knows might work. I have heard folks have used them in Zephyrs and they also have multiple magnets.

I hope that helps,

Joe

"A zephyr has 4 cylinders 2 coils which means 1 coil has to fire 2 plugs/cylinders at same time but one is one piston is in the tdc position and one in the bottom dead center. Since their is 2 coils on a zephyr 4 cylinders more than likely 2 lobes they alternate all that and don’t need the bakelite contraptaction you have in your flywheel"

I believe I made a mistake here in this statement. It should have been when 1 coil fires it fires 2 cylinders at the same time. Since their are 4 pistons 2 are at TDC at the same time and 2 are at BDC so it fires the 2 at TDC. Sorry for getting that part wrong but 1 coil is firing 2 cylinders at TDC at the same time.

On my 40hp Yamaha (CDI Ignition) outboard I rebuilt last year 1 coil fires both cylinders also. It is called wasted spark so both plugs fire the same time everytime. 1 piston is at TDC and one is at BDC. I sheered a flywheel key last year and the flywheel had slipped to right at 180 degrees from where it should be but it would still run just not correct. Took me forever to figure out it was a flywheel key.

I think your motor has no reed valves? and an unusual way of getting fuel (rotary valve) I think thats why it has the brushes are used to kill spark to the cylinder that doesn’t need it because piston is at BDC and fuel fumes are present. I think the zephyr also uses a rotary valve I have 3 of them and love them.

Did you ever determine what size condenser/capacitor value to use? Most the 1940 OMC smaller engines like the zephyr on the chart are showing .09 to .125 I wonder if by having less capacitance would that possibly help the situation?? Your using .2 just wondering by going down to .09 which would be half if that might somehow weaken the spark and allow the brushes to work better? You may still want to see if both plug wires fire with the Stevens Meter hooked up to coil and 1 not grounded with the .2 I suspect they will.

hope that helps,
Joe

I went and read another thread where you had posted this info:

-Coil Secondary 6.29 K ohms
Primary .8 K
-Kill Post working as designed
-Flywheel magnets – "So-so to Iffy"
-Flywheel brass grounding contact to "ground", "0 to .7" ohms resistance.
-The carbon brushes have good continuity to the coil,
and not grounded out anywhere.
-New plug wires with good readings at terminals.

6.29K ohms on the secondary which that is how I would test motor. Hook up multimeter to secondaries. Use the snaked wire to go to points connection using either a buzz box or multimeter slowly turn flywheel in the direction motor runs. Secondaries continuity should break before the points break. The secondary continuity also has to stay broken until points break. If all that checks out then bakelite is in the right position.

Hope that makes sense,

Joe

ps I also read where mumbles fixed a crack in one of his brush holders with clear fingernail polish. If motor still doesn’t run right you might consider coating the entire outside of both brush holders with clear fingernail polish that might help prevent arching if its happening.