Engine slowly loses power when in gear and driving after 20 mins of running from cold. Rpm drops from 2100 to constant 1200 rpm and remains at this rpm.
Restarting and running the next day - same result.
All filters changed twice all clean no crud.
Air filter renewed.
Secondary filter unit on engine has pump for bleeding this remains solid. Previously I found in the past if it was possible to push the pump in then there was air in the system.
Removing fuel fill cap on fuel tank makes no difference. Fuel tank is 95% full.
Exhaust elbow clear (replaced last year).