Just a thought,


If the installation of the return line was done without a check valve; and, the engine was run for any time (or a considerable time) in this configuration, the carburetors were getting unfiltered fuel through the orifice. Any crud coming back through the return line would be limited to the size of the orifice (0.040"/0.041").



The reason for the check valve is to prevent unfiltered fuel from returning to the carbs through the orifice and plugging the orifice or dirting the carbs since the tap for the return line is between the fuel pump and the carbs and the other end at the header tank is bypassing the gascolator and filters.


There may be at least some potential for crud having entered the carburetors.