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Yamaha FZ750 Genisis 1987. This is a European model that wasn't imported into the US. I am totally restoring the bike and it is presently in pieces with body parts in the paint shop and chrome bits at the plating shop. I keep having problems with the electrics. The bike runs well for about twenty minutes and then sputs and sputters and will not rev, just idles. If I turn it off it won't start until I let it cool off. Then it starts and runs fine again for about 20 minutes and then does the same thing again. I thought it was the CDI unit and replaced that with a used one. The Thai mechanic says he thinks it is a coil. I don't trust Thai mechanics as they have really screwed up this bike over the years. I am wondering if it is an ignition coil or the pickup coil. I have no idea what a pickup coil is or does but evidently I have two of them. Any insights, solutions or tests will be very welcome.

I'm no mechanic but that sounds like the pickup coils to me.

Ignition systems that use pickup coils are pretty hardy. Usually if something goes wrong, it's simply in the connections: a bad wire that has been crimped, a ground wire that is dirty, a place where insulation has rubbed off and lets the system ground out and stop working.

Usually the pickup coils are in one place (is there a little round cover on the right side on the motor near the bottom? 3-5 inches across? has a couple screws holding it on? I bet that's your pickup coil cover.)

It shouldn't be hard to access the coils (There will be tiny bit of oil under that cover, don't freak). Then you can follow the wires up from the pickup coils, to the ignitor (black box), and on to the ignition coils. You'll be checking for damage to the wiring the whole way. Since the tank's off already it should be even easier.

You would do well to get a wiring diagram for this machine and follow it so you know what you're looking at.

Looks like you might have to remove a frame piece to get at it? I can't find a good pic yet.

Does it idle smoothly when it starts to die? Or do you have to rev it a lot?
If it does that then I'd say it sounds like your carbs are way too rich. Otherwise yeah check your pickup coils, just like Coffee said!

Thanks for your input. As the bike is 23 years old a short of some sort is entirely possible. I have been told that my problems are connected to the coils somehow. I will check out all the wiring while the bike is completely in pieces all over my carport. I have wiring diagrams.
Sawasdi pi mai (Happy New Year in Thai language)

Coil(s) do sound like the next place to check, possibly the pick-up coils, but I'd check the ignition coils first. I've seen many coils work fine cold, then stop working when warm. It's actually very common. As they get warm and expand, the internal windings (an ignition coil is just a small transformer) can separate and break continuity internally.

When the bike gets warm and dies, have you checked to see if the cylinders are getting spark? If the spark is gone, it's obviously electrical and time to troubleshoot the coils. If the spark is still there, it could be the carbs giving you grief.