Ok, my 292 is has been showing some blow by out the valve cover breather lately. I decided to try hooking up a pcv valve setup to control the fumes. I pulled off the mercruiser valve cover and replaced it with the stock cover that has pcv provisions. Installed new 3/8 hose and a pcv valve for the 292. The idle went way up like it has a vacuum leak. Also it made the motor miss at idle and on deceleration. The idle also gets real rough when coming to a stop after going highway speeds so the miss is not a constant. The second I cover the valve with my thumb the miss goes away and idle shoots way down. So whats the problem here. I tried another pcv with same results. I was sorta thinking with the added airflow in the motor my carb might be off or lean. Any ideas welcome.

Could it be the wrong size PCV? Allowing too much air thru that valve. It can mess up the vacuum for the vacuum canister, causing a erratic idle.

Did you reset the idle after the valve was installed?