I have realised that after my merc's engine is warm to like 80C and standing idle, the engine begins to rev up to 2200 rpm then falls again to 500 rpm. Sometimes also after releasing the gas pedal the rev doesn't fall instantly. Also somtimes applying a full clotch triggers the rev to rise. I'm planning for a service soon but I need some advice first. Any ideas to what might be the cause? I need help.