Coil firing on one lead but not the other?

[Ola]

I'm pretty stumped here, although the evidence points in one direction. Bike is a 1986 GSX750ES

It started with an occasional stutter turning into a dead cylinder. #1 pipe was cold, #4 was fine. Compression is great across the board, voltage to primaries is good also. This is what I've done so far:

• Swapped plug leads between #1 and #4, the problem moved to #4
• Swapped coils, the problem moved to #2, same lead
• Swapped plug caps, the problem remained with the bad terminal
• Swapped leads between the terminals, the problem remained with the bad terminal

I've verified the firing cylinder by feeling the exhaust pipes, even though a dead cylinder's pipe will take some heat from the engine, it only takes a minute of high idle to almost get burned on a good pipe while still being able to grasp a dead one.

The good coil measures 24K Ohm between the plug caps, the bad coil measures 37K. The two different leads on the bad coil have identical resistance at about 4.5K Ohms. When testing the spark with an additional plug, it sparks but less than #4. Tried the original #1 plug, it too sparks but less than #4.

So everything points to the terminal being bad. I tried cleaning it, hard to see it inside there, but it looked fine, no change after cleaning. The accepted knowledge is that the coil should either fire on both or none, not just one. But this is clearly the case, unless there is something I'm missing. If I can fix this without replacing it, I'M ALL EARS! But by the looks of it, I'll give the junkyard a call tomorrow and see if they have one or two good ones at a reasonable price. Can't afford Dynas right now, can't afford the waiting either as I leave for the Alps in 11 days...

Any test or replacement check appreciated!

Bonus info: The GSX750ES runs pretty good on three pots! Maybe a tad vibe-y. So this is what a Triumph triple feels like.

[doctorgonzo]

hmmmm..........
If the coil fires one cylinder but not the other it HAS to be a plug wire/cap or a plug I think. On these bikes both cylinders fire each time the coil fires, one just doesn't have any gas in it.

[Ola]

hmmmm..........
If the coil fires one cylinder but not the other it HAS to be a plug wire/cap or a plug I think. On these bikes both cylinders fire each time the coil fires, one just doesn't have any gas in it.

As mentioned in the bullet points, the problem moves from cylinder #1 to cylinder #4 if I swap leads across plugs. There is fuel and compression all across the board. If I take the leads out of their coil terminals, swap them around and then re-attached so the original terminal for #1 is still firing on #1, the problem remains.

[rustybronco]

Swap plug caps from 1 & 4, or just measure the resistance of just the plug caps (they unscrew).

Bonus info: The GSX750ES runs pretty good on three pots! Maybe a tad vibe-y. So this is what a Triumph triple feels like.

or a Kaw triple without the smoke.

***edit*** OOPsss....

[*]Swapped plug caps, the problem remained with the bad terminal

a bad wire... dig it out and replace it.

[Ola]

***edit*** OOPsss....a bad wire... dig it out and replace it.

It's in the bullet list too. I have swapped wires and plug caps, alternately. The cylinder connected to the specific terminal remains the problem.

The wires have identical resistance, so do the plug caps unscrewed. I thought it was just a poor connection between the wire and the little spike on the coil terminal, but swapping wires eliminated that. Put it back original, make sure plenty of wire touches the terminal - problem remains.

[doctorgonzo]

So the problem moves with the plug wire if you leave it attached to the coil and just swap it to a different plug, but taking the plug wire off the coil and moving it to the the other coil the problem DOESN't move with the plug wire? Just trying to clarify (lead vs terminal vs plug wire gets confusing).

[Ola]

So the problem moves with the plug wire if you leave it attached to the coil and just swap it to a different plug, but taking the plug wire off the coil and moving it to the the other coil the problem DOESN't move with the plug wire? Just trying to clarify (lead vs terminal vs plug wire gets confusing).

Yes, this is exactly what happens. I haven't tried swapping the wire between the two different coils, just between the two different terminals on the 1-4 coil.

Disconneting wires on the coil, swapping them around but connecting same terminal to same plug = same cylinder dead
Simply swapping the leads around like you could do on the side of the road = problem moves, other cylinder dead

[Dave8338]

Sounds like a bad side on that coil.

[doctorgonzo]

Sounds like a bad side on that coil.

Or corrosion or something up inside the coil terminal stopping the lead from making good connection.

[Ola]

Or corrosion or something up inside the coil terminal stopping the lead from making good connection.

It has to be something like this. The terminals look identical, no rust or grime. No cracks in the plastic, but it could still be rusty on the inside. The unit itself won't open, so the junkyard is getting a call tomorrow morning.

[salty_monk]

Sounds like that coil has shorted out inside somewhere....

[Ola]

Update. New coil, ohms exactly like the good one, reused plug caps, firing like a champ on all fours.

Gonna be strange to go back to 750 cc now that I'm so used to 562.5 cc and a spare air compressor.

[Larry D]

Update. New coil, ohms exactly like the good one, reused plug caps, firing like a champ on all fours.

Gonna be strange to go back to 750 cc now that I'm so used to 562.5 cc and a spare air compressor.

Just remember to HOLD ON !!!!!!