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Join Date: Nov 2007
Location: Chandler, Arizona
Ok, it does not sound like your problem is caused by the black knob. All that does is adjust idle speed by holding the throttle open just a little. It sounds like your problem is in the pilot jets as lance328 said. If I were you, I would remove both pilot screws, and spray some Gumout into where the screws went. Then spray in a little WD40, and put the screws back in. Lightly seat them, then back them out about 2 1/2-3 turns. I have mine set at 3 turns, but have to turn them back in about 1 turn to get through the emissions test every year.
If that doesn't do any good, you will need to remove and thoroughly clean the carbs. You shouldn't have to replace anything, just a good cleaning with spray Gumout.
Even with the pilot jets completely plugged, you can usually get it to run on the main jets, without touching the throttle, by adjusting the idle speed (the black knob on the right) high enough. But that doesn't solve the problem. Don't be afraid to mess with the pilot screws, there's no rocket science involved here. As long as you get them back in close to the right number of turns, you won't mess anything up. Try that first, and see what happens. Jerry.
I am a motorcyclist, NOT a biker.
1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike