TOPIC:

Received an interesting e-mail today. Apparently the bike starts easily, seems to idle well and "pulls like a locomotive" on the highway but has a rough spot off idle and the 2 plugs on one carb are black/very sooty. Idle-air screws are out 4-5 turns and no adjustment to them makes any difference.

The "canned" answer is usually, ".... dirty idle circuit - take it apart and clean it again".

Here's the "twist", for your consideration: If the plugs are sooty (too rich) and you are not getting any changes in their condition or engine operation regardless of the # of turns on the idle-air screws then I would be highly suspect of the supply air to the idle-air circuit.

On either side of the carb throat intake are two holes. One on top of the other. These provide "mixing" air to the idle and high speed circuits for the venturi on that side. The lower hole is for the high-speed and takes mixing air to the emulsion tube. The upper hole is for the idle-screw bleed-air. If you look into these air ports you will see there is a metering insert at the far end. Although these metering orifices are large at +.045" they are none-the-less restrictions which could get blocked. The owners claim that both idle-air on the same carb (and only this 1 carb) are demonstrating the same problem is certainly very coincidental.

If the idle air supply is blocked then there can be no ability to adjust the idle fuel/air mixture. Hence the plugs are black and very likely the cause of the (over rich) roughness from "off idle".

This could be why we frequently hear people suggesting they can completely remove the idle-air screw with little to no change of idle RPM, given everything else is OK.

If this is the case then some further cleaning is required. To what degree the carbs need to be disassembled and how the cleaning is accomplished will be up to the user but given the scenario this is the FIRST place I'd start at. I'd be willing to bet the blockage is "insect" related.
Failing that: "someone" forgot to remove "something" from these holes which may have been acting as a deliberate plug, during a previous cleaning process.

Hope this helps. You know who you are. Let us all know. Inquiring minds need the stimulation !

Note: I took liberties with some "generalization" merely to keep this short, simple and to the point for clarity.

And what are the chances that "somebody" left out the idle circuit jets?

kawboy wrote: "And what are the chances that "somebody" left out the idle circuit jets?"

Always the devils advocate, kawboy.

Anything is possible. And in this case the "punishment, would fit the crime" !

....and the longer I sit here pondering that possibility, the more I can visualize it happening. It is just as likely (maybe even more so) then the two idle-air supplies becoming simultaneously blocked.

Good call kawboy !

SO: (on behalf of kawboy) "DID YOU FORGET THE IDLE JETS"?

For small jobs I have occasionally sorted and separated small bits and pieces into ice-cube trays but that's not without it's draw-backs if the tray gets bumped. That's why I prefer to use "BAGGIES". Various sizes available for pennies at a $ store, impossible to lose and with a cork-board on the work-area wall I can "push-pin" those "little baggers" out of the way, have all the "bits-n-pieces" in-site, labeled, in order and immediately available.

Thank you Scotch and Kawboy for your input and suggestions, as I believe that it is my issue being discussed here, I can assure you that the Pilot jet / idle circuit jets are most definitely in.