Just a thought - I had almost exactly this problem - turned out that the
carb throttle bodies had been rebushed, but they had been drilled all the
way through, with the bushes only pressed half way through - this leaves an
internal leak around the throttle shaft. Of course, it's not obvious from
the outside of the carb, but it lets a stack of air through on idle, and the
overun. The bloody thing would never run quite properly.
Needless to say, the shop that overhauled those carbs is not on top of my
recommendation list....
Tim
66 B rdstr YGHN3-3238
<snip>
Next best bet is that the throttle shafts are worn so the butterfly valve
may be hitting the side of the throat and stopping before it closes all the
way. Wiggle the shafts up and down to check the wobble. They should move
very little, hardly any perceptible amount at all. If you get anything
like 0.010" motion there you need to renew the throttle shafts, and maybe
consider drilling or rebushing the throttle bodies while you're at it.
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