Today I decide to go to have a Mot test. I didn't use my 3w for a very long time but it started fine. After 5 minutes of idling it starts to idling very bad. Like 1 cylinder was missing. I cut off the engine and get back into the garage. I just buy new spark plugs and tomorrow I will put new gasoline in the tank. No engine light. The engine start fine but the idling is very bad and it I push the throttle the engine doesn't run fine. Fuel pump noise seems fine. Any thoughts ?

Morgan 4/4, Caterham Seven, Ginetta G27, TVR Chimaera