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If single plane manifold the entire engine may be idling on the one idle circuit (passenger in your example). YOu may also be able to richen the drive side and then crank down the passenger side and have the same effect.
You could have a blown power valve. You could have a clogged air bleed.

Is it idling rich???



Both screws should be turned out the same amount. Start with the screws about 1-1/2 turns out. Turn them in or out 1/8 turn at a time. If you can turn them both in all the way, then either you have a clogged air bleed, air bleed is too small, or the idle feed restrictor is too big.