Auto Parts: vacuum modulator, holly carburetor, vacuum modulator

Question
will the vacuum modulator make a motor run funny? i did a motor swap. it has a 350 4 barrel holly carburetor 650. it has a dead spot when you open up the throttle to 4 barrel.

Answer
    The 650cfm Holley is usually a mechanical secondary carburetor.  They are recommended for manual transmissions only.  This doesn't mean they won't work with an automatic, but they will have to be tuned by someone knowledgeable to get them to respond correctly to the throttle opening.

    Now, the vacuum modulator is the thingie on the transmission that controls the shift points, along with the governor.  If it has a leak, or any other good-sized vacuum connection does, there will be a lack of throttle response due to the lack of vacuum at some point during the throttle's travel.

    My guess is that it's the carburetor needs tuning.  When you start to open the secondaries you are not getting the proper amount of fuel.  Check a spark plug to see if it's running too rich.  If it's not, then you'll need to use a bigger air bleed or a different power valve.  Holley sells a nice little "how-to" tuning book which is carried at most speed shops and by places like Summit.  It should get you to where you want to go.  The other option is to replace the carb with a 600cfm vacuum secondary carb (#1850), which should run well right out of the box.