If you had a fuel filter installed, why would it be dirt in the carburetor?
Those who frequent these forums know the Rochester B is certainly not my favorite carburetor; carburetors (even Rochester B's) fail over 1000's of miles or a couple of years.
On the other hand, ignition systems fail from instantaneously to over a period of days.
Yes, it MAY be the carburetor, but I would investigate the ignition system first. A perfect carburetor will not run an engine with a defective ignition.
And as Carl is suggesting, you may have a fuel delivery issue (fuel pump diaphragms don't care for ethanol fuel).
Try filling the carburetor with fuel through the bowl vent. Then try to start the engine. If it fires, check the fuel pump. If it doesn't fire, try pouring a tablespoon of fuel into the throat of the carburetor; then close the choke and try to start.
If it fires with the fuel down the throat, but not with filling the carb, then its time to rebuild the carb.
If it doesn't fire either time, its time to overhaul the ignition system.
Jon.
