9.8 Running on One Cylinder Only

[JDusza]

9.8, 2 stroke. Serial A901055. Top cylinder not contributing. Compression at 125 psi, spark plug does fire and spark energy will jump 3/8" air gap. With the engine running I can pull the spark plug lead from the top cylinder and the engine performance does not change in the slightest. If I pull the plug wire from the lower cylinder the engine will die. If I pull the bottom first, the engine dies. It will not run on the top cylinder alone. So, top cylinder not contributing. Compression is present. Spark is present.
Tried spraying premix into top cylinder and I get nothing. I believe it should fire. For this test, "timing" shouldn't matter. With compression and spark I should get a reaction. My conclusion is that the top cylinder spark-to-compression timing must be out. Bottom cylinder spark-to-compression timing must be correct or close enough.
What parts influence spark-to-compression timing or cylinder-to-cylinder timing; flywheel magnets, crankshaft design and machining, magneto coil-to-coil positioning under flywheel, trigger signal, switchbox?
Do stator and magneto coils influence cylinder-to-cylinder performance other than by their positioning under the flywheel? With the flywheel magnet they generate the stuff to be processed but can they influence cylinder-to-cylinder? I understand the switchbox sets a threshold and discharges capacitors to the ignition coils, but, does it have an influence over cylinder-to-cylinder discharge relationship?
Can I mark the flywheel based on TDC of pistons and put a timing light on each plug lead?
Without proper diagnostics I'm doomed to throw money at parts and probabilities.
What part should I go after first?
Thanks,
J

 

[racerone]

Put your own timing marks on for top and bottom cylinder.---Start motor and use the timing light.

 

[JDusza]

racerone, nailed it! Thank you for sticking with me!
Found the factory timing marks on the flywheel and a reading window on the recoil starter cover. Verified flywheel timing marks corresponded to TDCs. Repainted marks for a good read.
Started the engine. Lower cylinder timing mark reads nice and steady. Moves slightly with throttle up advance, makes sense. Top timing mark is nowhere to be seen. I know there's energy in the light pickup because the strobe is flashing.
Bad trigger.
Thank you, racerone.
J

 

[racerone]

And how did you determine it is a bad trigger ?

 

[JDusza]

The bottom cylinder parameters, the basics, must be "close enough" because it is running. Bottom ignition fires "close enough" to bottom TDC. Verified with timing light attached to the bottom spark plug wire.
Switched the timing light to the top cylinder spark plug wire and its timing mark is not seen. I believe there is an ignition pulse detected by the light for the strobe is flashing. I moved the light around the starter cover as best I could but cannot find its mark. Maybe I'd have better observation luck if I mark lower on the starter's ring gear. I'll have to look.
My conclusion is the top cylinder timing mark is showing up somewhere underneath the recoil starter cover where I can't see it. The top cylinder is being told to fire at some "out of spec" time. It's not firing on its compression stroke. The bottom is working so it cannot be that the top cylinder is firing properly and the bottom is out of phase.
If the flywheel magnetic fields were messed up the bottom cylinder wouldn't run so well. All signals would be all messed up. The magneto coils are fixed in position, they can't move. The name switchbox implies switching something so that's still a possibility, I suppose. The little I know is that the switchbox accumulates and dumps energy to the ignition coils when the trigger says to. The gear after the switchbox has been swapped around to verify it's all working.
The only other way I can think of to verify the trigger and switchbox is to analyse their signals directly. I could see energy pulses with a meter but I can't determine the dwell between them. I need different test gear.
I do admit what bothers me is the failure mode. How could the trigger fail? It's just a sensor and wire ...
J

 

[racerone]

So does one trigger coil fire both cylinders ??

 

[JDusza]

Ahhh. You're making me think.
There's plenty of forum input for non-existent spark but the population isn't coming through with how to deal with a malfunctioning spark. Nothing meaty on this or my previous post. What is your experience telling you? Where am I going wrong? This can't be the only Merc with this problem.
From what I can see, there are three things influencing the spark-to-spark timing; flywheel magnets, trigger and switchbox.
Flywheel magnet could be a problem. It is not two independent magnets. The magnet is a continuous circle of material around the entire flywheel. It must have been positionally magnetized during manufacture. If the fields got then the trigger won't be pulsing at the right time. How do I test a flywheel magnet? Can I monitor the trigger with a meter and rotate the flywheel past it quickly enough to create a pulse but slowly enough to physically observe flywheel location of said pulse? I think the trigger is just a Hall Effect sensor, right? Would expect to see two pulses through one revolution of the flywheel, 180 degrees apart. I'll have to try this.
Switchbox: I have not found a good explanation of what exactly a switchbox does, given the name. I understand they accumulate and release ignition coil energy but that's the limit of my knowledge. Why is called a "swtich" box? What is it switching? Maybe the Mercury design team says the capacitive discharge is the result of a switching function.
Then we're left with the trigger. I don't understand how they could go bad. If it's a Hall Effect sensor, there's nothing to fail. It's just a sensor with a coupe wires that will generate an electrical pulse based on magnetic strength.
Maybe I'm off target. Maybe it is the flywheel.
racerone, you're loaded with experience. What is this scenario telling you?
Thanks,
J