The miss could be either fuel or ignition.
If it is the fuel, it a stumble like that would probably indicate an excessive
LEAN condition. If it is transient and goes away shortly while still at 2500
rpm, then it might be inadequate dashpot damping. If it is constant, then it
is less obvious. An air leak could be a cause.
More likely it is ignition. Maybe a bad condenser, bad/maladjusted points,
improper timing or the like.
First check that you have oil in the dashpot, and that you have the carbs set
correctly by using the lifting pins. If they are not lean, then look to the
ignition. Ignition can be a bugger to track down. I usually check/replace
parts starting at the plugs and ending at the coil.
An inductive timing light can be used to look for a bad wire. Use the pickup
on each wire and look for a rythmic flash. Any one wire that is significantly
more irregular could suggest that this wire is bad. A good flash in each does
not PROVE it is good, however.