I think that you have found the problem with the fuel line. the f plug is the best one and if you are not running a fuel filter put one before the carb 4 cycles are prone to get dirt in the carb wich will cause irregular runs and don't mess with the low speed neadle unless you know what you are doing !!!!!!!!if you have played with it let me know and I will send you a description of how to tell if it set right and how to adjust it if it is not
I have 2 150 saitos and they are good runners but I like the os better and if they had a 150 to 180 range single cylinder I would have one or two!



Ron mcgarth