It sounds like a burnt valve, or a piece of carbon managed to get between the seat and valve, which will require you to pull the head. you might be able to pull the spring and rotate the valve a couple times and see if that makes a difference.
I have had great success removing carbon deposits which drastically improved idle with. . . Water!!! My 92 Cherokee that was drinking oil for far too long due to bad valve stem seals, and a 97 Altima that was just a pos. sea-foam works good, but water works better, the added benefit is no smoke, and its cheaper if that matters. If using water be sure to use a misting garden sprayer so you don't hydro-lock your motor!!! The engine being at operating temperature is also critical to a successful cleaning. Sea-foam is safer if pouring, otherwise water in a misting sprayer works better.
There are a couple things you can do to replace a valve spring spring or valve stem seal without removing the head. I have done it both ways successfully.
Remove the rockers on whatever cylinder you are working on and.
Use a 20+ gallon compressor connected to the spark plug hole with a compression tester fittings, dump as much air into the cylinder as it will take. Take a socket with the same OD as the spring retainer and smack the spring a few times to loosen up the retainers and keys, I had to use a surprising amount of force and smacks to loosen up the retainers on my old Jeep. then use the proper tools to remove the spring, retainers, and keys.
The second way is drop the piston to the bottom of the stroke and shove about 5 feet of 3/8" nylon rope into the spark plug hole, then rotate the crank by hand to compress the rope against the valve. then remove the spring etc. . .