Can a coil fail only when hot

[Goggles Pizano]

04 620 with 01 900 engine swap, bike ran fine for at least 3 months.

Started out would stall at idle and would smell gas.

New battery and plugs, seafoam in the tank.

Now after it warms up (220+) for a moderate ride (25 mile commute @ 80-90 mph) it will idle rough and I can smell gas coming out of the exhaust.

Everything is fine when cold or just riding around town.  My plan is to bring it in to Ducati for a TPS reset and TB sync.  Wondering if I should replace the coils first to see if that fixes it.

Thanks

[Speeddog]

Find out which cylinder quits running, swap the coils and see if the problem follows the coil.

[koko64]

+1
With a hairline fracture in the coil winding, as it heats up, the winding expands and you're running on one. Well that's how it was explained to me. Had it happen on a couple of bikes over the years. From memory it suddenly shut down the spark.
Gave me a good excuse to buy performance coils.

[ducpainter]

In my experience coils always fail when hot.

[avizpls]

just to concur, YES its possible (even likely) and swap them and see.

[Goggles Pizano]

Thank you very much for the advice/info!

I Tested coils cold.
For an 04 620 stock coil resistance is:
Secondary 13.5 KOhm +/- 20%
Primary 4.5 Ohm +/- 15%
Cap 5KOhm +/- 10%

Actual readings:
Secondary 18.9 on both coils
Primary 4.8 on both coils
I don't know what they are talking about with the cap cause there was no picture  

So, I have a few questions.  Since my problem only shows up when the bike is hot, do I need to test hot?
The 18.9 on the secondary exceeds the mfg specs of +/- 20%, does that mean I need to change both of them?
and finally,
Was the key supposed to be on during these tests?  

Thank you again

[suzyj]

So, I have a few questions.  Since my problem only shows up when the bike is hot, do I need to test hot?
The 18.9 on the secondary exceeds the mfg specs of +/- 20%, does that mean I need to change both of them?
and finally,
Was the key supposed to be on during these tests?  

It would be a good idea to ride for a bit and then test them.  Your coils shouldn't get particularly hot though.

The reading you gave for secondary resistance doesn't sound bad (assuming it was 18.9K ohm.  If it's 18.9 ohms, then bin it.)  The usual failure mode for wound things like coils is for them to be completely open circuit due to a broken winding, or else to have a lower than normal resistance due to a shorted winding.

You shouldn't be testing resistance with the coil in the bike, as the circuitry that drives the coil primary will affect the resistance reading.  So no need for the bike to be on - pull the coil out instead.

And +1 on avisplz's advice.  You have two coils - swap them over and see if the fault follows the coil.

[Goggles Pizano]

Update
I just changed the spark plugs and wires increased plug gap to .030.  The bike starts more easily, I ran it up the highway for a bit and heated it up and all seems fine after 23 miles with an engine temp of 247.  I ALSO discovered that one of the fuel lines was resting on a cylinder head.  I moved that out of the way as well.   Don't know which one helped, but so far so good.

I tested the resistance of the old wires.  They are stamped with 5000 Ohms, vertical tested over and the horizontal (which showed signs of corrosion) tested 4.somethingk, but was not far off...  (This was a cold test)

A warm test of the coils with new wires showed a secondary resistance of 18.5k on the vertical cylinder and 19.26K on the horizontal, which are both out of spec, just one is getting worse... (figures it's the one that is a pregnant dog to change)

Also with the coils unplugged from the bike, the secondary windings showed 0.0 resistance. You got me there, hooked up they work /shurg...

The plan is to ride the bike to work tomorrow and see if I can bring the problem back...

Thanks again for all the help!!