The fuel system operates under very low pressure - gravity cannot provide much pressure when the distance from the float bowl to the top of the fuel tank isn't much more than a foot......the bottom of the fuel tank less than half a foot.  With a full tank the pressure at the carb is less than 1/3 psi......with an nearly empty tank it is less than 0.15psi!

ThumperPaul wrote on 01/30/24 at 10:49:19:


The fuel line and factory petcock/carb nipples are large enough that that an air bubble can freely move upstream and exit the fuel system.  The small fittings on the fuel filters are small enough that the bubble can be trapped and it will not freely exit....the fuel wants to come down and I believe the surface tension of the fuel is stronger than the small air bubble - and the flow stops.

This situation evidently can't be resolved in normal operation, and some members have even had this occur after using a fuel filter for an extended period.  When they first install the new fuel filter the tubing and filter are empty - and the fuel flows down the tube and pushes the air out the bottom.  After that the fuel system is subject to periods where the bike is not being stored and air gets into the system from the carb end and the air travels up the tubing until it reaches the small fuel filter inlet.....and that is where it gets lodged and stays.  The rider is suddenly at a loss as to why his bike has stopped running, and he starts looking for electrical problems and carb problems that don't exist!

Your fuel petcock has a fuel filter screen in the tank......and the carb has a fuel filter screen on top of the seat for the carb needle.  If both of those are in place and working....I really don't see the need for a third filter - especially when they can be problematic.

Over and out.....if you choose to run a filter - you have been warned!