sounds like a vac leak I had the same problem with my ltft's being at 16.4. First it was my s/c gasket fixed that and still had a leak. Ended up being my oil dipstick was broke right were the o-ring is on the inside. Now they sit around 8 at idle but i also added rockers and a ic so i have to tune it now. It is normal for the idle to change when you take off the oil cap. Keep trying the carb cleaner and see if anywhere else is leaking. Also I was told if you scan your IAC and they are under 50 you have a leak.
