I think Gaz makes a couple of good points worth addressing. Are your lifters hydraulic or solid? I'm thinking hydraulic and some failure or leak down is causing the bad valve adjustment. An easy fix would be it is just loose and out of adjustment. Start by adjusting it. Undo the lock nut and adjust the screw in the centre with the valves timed correctly. You can usually hand crank the motor over till a valve is at full opening. Then you would check adjustment on the other valve of the same cylinder. They should be adjusted till there is no play in the pushrod. At that point most hydraulic lifters require an additional 1/2-3/4 of a turn. I like to do that with the distributor out with a socket in a cordless drill so you can pump the
oil up to pressure so the lifters are more like actual running conditions.
If the lifter has failed you could visually inspect valve operation with an old set of valve covers with the tops cut out. The rockers are on a short shaft for each cylinder. They have a socket screw holding each end down. I don't think you want to remove them just yet.