Thanks for your answer.
Yes, I did remove the valve covers, thank you for clarifying. Glad to hear oil there is normal.
What is the easiest way to remove the carb?
After further inspection, I found a lose washer in the throttle mechanism? (assembly right beside the carborator) that seemed to be choking the engine. After I removed the washer, the engine fired right up! I have no idea where the washer came from and there is no visible sign of where it belongs.
Problem is solved, thanks for your great help!