Have you removed and cleaned the rocker arm assembly?
Keep the parts in-order when you clean them.
Clean/clear the hole in the top of each rocker-arm.
Clean the two tubes/pipes - clean from each tube's open-end to its closed-end (and clean/clear the holes on the bottom side of each tube)?
When you have the rocker arm assembly off, remove (and number) the push rods and then crank the engine
- does oil come out the oil-feeder tube to the rocker arms?
- do not do this until someone else agrees this would be OK/safe -
- or, wait until someone else yells "that's a stupid idea" -