Run some steam through it from a wallpaper steamer or a kitchen cleaner steamer device .
Remove the pipe from intercooler where it joins the manifold and place the steam pipe facing upwards towards inlet about two inches below it and the engine can take the steam it can pull in and no large drops of water can go in as they fall to the ground . Run around 6-8 litres of water as steam doing this and you will unstick piston rings and anything else such as valve stems gunned up.
Run the engine at a fast tick over say around 1100 rpm and change oil afterwards although I always found this was not needed but if the engines breathing heavy then it’s best to do it.
Result will be engine runs much better and this should clear the problem up if it is not a mechanical issue.
Do this before every oil change if you run veg
Just realised you are non turbo so just rig the steam up in a similar fashion so it’s drawn in by engine rather than forced.