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 ?

First thought would be a loose spark plug cap - the screwed on nipples on the top of the plugs are prone to loosening off which can give the kind of symptoms you describe. Tighten them up with a pair of pliers then get the plug caps back on securely and try again.

Hi Calum. thank you for your suggestion. I just put 2 new spark plugs and it seems to idle fine. Tomorrow in the morning I will check if it's all ok also after some minutes with engine on . Thank you for your aid

Does it have the urban cooling? If not it might have heated and went into safe mode.

An additional possibility:

I believe it's possible that after "5 minutes of idling" that the engine could get hot enough to activate the "skip fire" mode wherein the engine fires alternatively on one cylinder then the other.

Air cooled engines are generally not happy to idle for extended periods unless there is some sort of significant, external airflow.

I had the exact same symptoms. One of the nipples had loosened. Tightened it as much as I could while on the road. Later I took the nipples off, added a bit of hi-temp loctite then while tightening everything back down I crimped the nipples hard! Those are not coming off for anyone.

Problems went away.

Thank you all for your help. Tomorrow I will check any suggestions. I hope I will mot my 3w soon. Thank you all for your aid