Vibrations when low on fuel?

[jfoster]

I've noticed that when I'm low on fuel (1-2 gallons), the engine picks up a vibration at different rpm ranges, similar to the carbs being out of sync. When I fuel up it goes away and runs very smooth. Could it be vent related? Thanks in advance.

 

The following has been done:

 

Fuel filter replaced this spring. New plugs (pulled the tank). Sea foam treatment.

[Flyinfool]

Is it possible that the fuel pup is weak or dead and you are just running gravity feed. As the fuel level in the tank drops the fuel pressure and flow to the carbs would drop also.