I have a string trimmer that will start and run for 5 to 20 seconds then stalls. I have replaced al the fuel lines, fuel filter, spark plug, replaced the diaphram and cleaned the carb. I have also tried adjusting the carb and nothing seems to make a difference. It seems like it just burns the fuel in the carb then stalls. It seems like it is not pumping it's own fuel but the new diaphram should have taken care of that, well it did on my chainsaw when it was doing the same thing. Is there anything else I cant or is it time fr a new one? It is a 31cc 2 stroke Ryobi 775r.