does your weber have a vacuum relief valve to reduce the
manifold vacuum when you snap the throttle shut? Does it have metering
rods in it? If you are backfiring you are getting unburned gas in
the exhaust system. Whether or not it is caused by too lean a mixture
or too rich a mixture is open to debate. It would seem that your backfires
occur when you shut the throttle plate and get a very high manifold vacuum.
This makes me think you are getting too rich a mixture. A change in the
exhaust could cause your carb to need adjustment. Basically I agree with you
except I think you may be too rich, not too lean. Misfiring under hard
acceleration and backfiring would seem to support to rich a
mixture also. I think the weber has an accelerator pump which squirts
gas into the manifold to make up for lost vaccum. It would be pretty hard
for that carb to run too lean under acceleration. Of course, anything
is possible.
woodruff@caen.engin.umich.edu
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