<VV> Intermittent vacuum leak?

[Mel Smith]

My '65 140 with two carbs has been acting oddly lately. I drive it around 
and at some stop lights the engine will bog down and not accelerate, while 
at other stoplights it seems perfectly healthy. It seems like at random 
intervals the engine will refuse to rev up and will kind of kick back when 
you open the throttle. I have already rebuilt the carbs and have since 
cleaned them out thouroughly. The theory presented to me has been that there 
is a vacuum leak causing the hesitation, which would explain it since it 
sometimes idles poorly, but how could the leak only randomly affect the 
engine at odd times? It doesn't seem to be related to the temperature of the 
engine. What are some sources that can act like this and what are other 
problems could it be? I'm running plates to cover the secondary carb mounts 
and there are seals on the throttle shafts.

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