Assuming you are generating the serial IRQ stream to the CPU correctly, if you are using IRQ7 , then you need to make sure the IRQ 7 isn’t being assigned to any PCI devices. You need to eliminate and IRQs you want from the LPC  from the PIRQ table.

There are also some configuration of the 8259 for level vs edge triggering.

Also, it is handy for the SERIRQ to be in continuous mode.

Sincerely,

Anthony A. Marchini
Software Engineer