Don---It's somewhat unusual for the engine to go all the way up to 3,000
rpm when both throttle plates are closed. Assuming that there's no
linkage hangups, I don't think a blown intake manifold gasket alone
could cause this much of an increase. Some, maybe, as the engine has a
new supply of air in which to breath. This being the case, the spark
plug on the cylinder seeing the extra air should be snow white.
More likely the bi-pass is blown on at least one carburetor, and is
admitting air (and fuel) thru this passage. It is still unusual even
when this happens for the "idle" to stay this high. Could be a
combination of some of the above.
Dick
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