The engine ran great!!!….except when it didn’t.

[fleetwin]

Seems like there is some sort of ignition issue, possibly aggravated by moisture….
I guess I would do a spark check on it when you start it and it is running like crap…Perhaps use a timing light or those inline neon testers to figure out which cylinder is misbehaving…
Not for nothing, and perhaps I skipped over it in your post, but have you tried new plugs? Just cuz the ones you have in the engine are relatively new, doesn’t mean they are any good…
The only other issue I can think of is if this engine has a stop circuit/key switch, or perhaps one you have added to the wiring…If so, you may want to disconnect the leads (if that can be done easily without removing the flywheel) and retest…

[nj-boatbuilder57]

quote Tom Manley:

You need a high dV/dt too.
Tom

That makes sense. Since this is a short circuit type of discharge, I’m thinking that the energy content (Ko) = Vchg^2 / (R*C), but I don’ know the charge voltage.

Any idea?

Fleetwin: The kill circuit is just the standard "points wires", run to the boat’s dash, to a SPST switch. For one of the wires to ground out but not the other (i.e. kill just one plug) isn’t really possible. That said, I’ll be inspecting everything when I tear down the mag plate.

[outbdnut2]

Instead of all the work of cobbleing in a capacitor meant to be soldered in electronic equipment, just buy Sierra # 18-5199 cap. It”s a direct replacement and I’ve never had a bad one. Order online or with same part number at NAPA or O’Reilly auto parts stores.

Note that the leads on those other capacitors are small solid core wires that can break with vibration flexing them.

At approx $5 each, it’s not worth my time to fit in something else.
Dave

https://www.iboats.com/shop/sierra-condenser-18-5199.html

[Mumbles]

quote NJ-boatbuilder57:

Why the 850V rating, though?

Because one of our electrically gifted members went to great lengths with the tech department at Digi-Key to come up with specific caps which will work and hold up to the demands of the universal magnetos used in most of the OMC motors. Depending on their construction and ratings, others may have a very short working life and these ones were found to be the most reliable. The same cap with a lower voltage rating might work to but for how long? It seems these mags have very high voltage spikes and they felt a cap with the higher rating would be best in the long run. 850V might be a bit of overkill but at least they won’t let you down two miles from the boat ramp.

The .22 uF Orange Drops I’ve used for outboards are fairly large and there’s not enough room for them on the universal mag plates but they fit quite nicely on the larger mag plates of the prewar motors.

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Attachments:

[chinewalker]

Is it possible your fuel tank is pressurizing in the sun and forcing fuel into the motor through the carb float needle? You mentioned unburned fuel on the water. Standard OMC steel tanks are vented to let air IN and fuel OUT, but they don’t free vent. They’ll build up pressure in the sun.

[nj-boatbuilder57]

I keep the cap cracked to prevent just that….

[fleetwin]

quote NJ-boatbuilder57:

quote Tom Manley:

You need a high dV/dt too.
Tom

That makes sense. Since this is a short circuit type of discharge, I’m thinking that the energy content (Ko) = Vchg^2 / (R*C), but I don’ know the charge voltage.

Any idea?

Fleetwin: The kill circuit is just the standard “points wires”, run to the boat’s dash, to a SPST switch. For one of the wires to ground out but not the other (i.e. kill just one plug) isn’t really possible. That said, I’ll be inspecting everything when I tear down the mag plate.

The only thing that my lack of electronics has taught me is that "anything is possible"…Sure, not likely that a frayed/grounded lead would affect just one cylinder, maybe there is a very slight short to ground affecting both cylinders…Just saying might be easy to just disconnect the stop switch leads from the mag plate connectors and restest (if they are easily unplugged without pulling the flywheel/mag plate).
You may want to try using the neon inline testers or timing light (after sunset) in an effort to isolate the problem cylinder before ripping the mag plate apart again…

[nj-boatbuilder57]

Well, folks, here’s the deal. I am completely, totally, 100% UN-able to make this engine run badly.

I’ve bucket run this thing for the last few weeks. I’ve spritzed the wiring with a spray bottle full of water while running, trying to get the ignition to arc out. I’ve run it in the dark to look for arcing. I’ve pulled the flywheel & inspected everything. Looked for arc spots and/or carbon spots. Replaced the caps, too. I’ve disconnected & reconnected the mag kill wires. All of the wiring is 100% new, but I’ve re-traced everything anyway looking for anything suspicious. I’ve removed & re-seated the spark plug wires/boots too many times to count. This engine starts easily & promptly, and runs well.

Measured the spark energy with a tester: constant & strong, averaging around 37kV.

I suppose I shouldn’t complain about a well-running engine, but there’s no denying that twice (on an otherwise perfect 2-week vacation of daily use), there was an ignition issue with what I suspect was one cylinder not firing. Hard starting, rough running, lots of un-burned gas in the exhaust. It happened……yes, it did. But it wont now.

Frankly, the ignition system of a 2-cycle magneto-based engine is a pretty simple device. There’s just not that much to go wrong…and I’ve done everything I can think of to tease out a problem.

Thoughts?

[outbdnut2]

quote NJ-boatbuilder57:

Well, folks, here’s the deal. I am completely, totally, 100% UN-able to make this engine run badly.

Thoughts?

Maybe something in the gas fouled out a spark plug?

I hate those intermittent problems that happen infrequently. I have a 1991 15HP Johnson that I don’t trust for that reason. Twice in 5 years, it has not started for about 40 pulls, but perfect all other times. It has mega-hours on it from a failed resort.
Dave

[nj-boatbuilder57]

Could be. That said, I’m bucket testing with gas that’s leftover from the same trip, so if there’s something in the tank, it has yet to reveal itself.

At this point I can neither rule "in" nor rule "out" anything at all. There’s nothing "obviously wrong" in the ignition system that I can find.

[Author]

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