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Idle circuit cleaning


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John Dickens

There is no idle circuit as such. It's not like a carburettor. The idle mixture is controlled by the Motronic unit and the idle speed is controlled by the Bypass screws on the throttle bodies. The most likely cause of rough idling is the need to re-balance the throttle bodies.

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My 99 RT is idling roughly and it was suggested I may need to clean the idle circuit. Anyone have instructions on how to do this?


rondob4, I presume you are referring to the idle air bypass circuit? Yes cleaning that area can improve the idle & increase idle stability..


Basically you have a big brass screw on each throttle body (facing rearward & to the outside).. To do it correctly just pull the brass screws out & clean the tips with carburetor cleaner or brake clean.. Then spray some 02 sensor safe carburetor cleaner into the screw bores a few times to clean those out (a little low pressure air into those screw bores also helps).. Once the screws & bores are clean, lubricate the “O”” rings on the brass screws & re-install them… Turn the screws in until LIGHTLY seated then back them out 1-1/2 turns from seated.. You will now need to use a balance device like a home-made manometer or Twin Max to balance the TBI’s side to side then re-set the idle speed to specs..



Or if you don’t have the means or expertise to re-balance the TBI’s you can do the above cleaning by PRECISLY MARKING those brass screws for position,, then precisely counting the turns until lightly seated (write it all down) then removing the screws & cleaning the screws & bores,, then re-install the screws to EXACTLY where they were before removal.. This only works if the TBI balance was correct before the cleaning process..


It’s also possible you have a partially plugged fuel injector. Maybe try running some fuel system cleaner through your next fueling cycle.. Any of the TECRON based products seem to work about the best.. Also works best if used before a long fast ride..


If you haven’t adjusted the valves in awhile maybe do that also.. A tight valve can really foul up the idle quality..






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rondob4, I presume you are referring to the idle air bypass circuit? Yes cleaning that area can improve the idle & increase idle stability.




Thank you, exactly what I was hoping to hear on the forum. I've tried running the Tecron cleaner thru, but that didn't seem to do the trick. I'll try your suggestions.

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