I have an annoying idling problem with my '78 55 HP Johnson outboard (see Other topic).<br /><br />It seems one or two of my carbs have some kind of problem, since I think it's running too lean at idle, but not always. I've cleaned and rebuilt the carbs dozens of times, so I'm out of ideas here. It might be an air leak somewhere or a float that's stuck sometimes, or something else.<br /><br />I have two brand new carbs of a '77 50 HP rope start model (part no. 322990). These carbs have adjustable idle mixture orifices. The carbs on my 55 HP (part no. 321700) have fixed idle orifices.<br /><br />My question is: are the carbs exchangeable and are the idle mixture orifices the only difference? (apart from the size of the high speed orifices, which I should exchange also).<br />If so, can I try these carbs and see if there is improvement?