86 fj 1200 running issue

[ren-dog]

Do any of these FJ's have electronic fuel pumps?
If so,a faulty pressure, could be a contributing factor too.

[RichBaker]

3) Check your gas cap to make sure it's allowing air into the tank and not creating a vacuum as gas level drops while being fed to the carbs.
With my '84, I was losing vacuum intermittently at my petcock and the petcock was shutting off fuel to the carbs....What a pisser that was...Took me a while to figure that one out...

A vacuum lock inside the fuel tank
Is this common on the 84?

Nope, it's part of the petcock ass'y. These have 3 positions: PRI(me), which flows continuously; On, which flows when engine vacuum is present; and RES(erve), same as ON.  I don't know if it was Federally-mandated, but the manufacturers started using these in the late '70s.
They come on every road bike (that uses a petcock) I've seen in many years....