hey evan you say that it was missing a screen and now it dont work after replacing the screen them zama carbs can be very very tricky and normally it takes a special tool to adjust those carbs since mtd owns ryobi well i say if its 8 years old ryobi was on its own back then but mtd bought them out and i dont know you may be needing to replace the carb itself i can diffently tell that they dont last forever them carbys dont only a few years normally expecally if they get trash inside them it starts working on the inner parts of the carb besides bruce i m,yself am in the outdoor power business been in it since 1988 but im here in the usa and you all are over in australia i think i spelled it correctly i hope but any way it may be time you get a updated carb and give that a try i not sure if bruce has got mtd parts or not but if not im sure you can find it
i have worked on them 725R models tons of them and they can get tricky and make sure you changed the fuel filter and also make sure the fuel lines are in the correct spots the primer jet should be where you have the line with the fuel filter attached goes in at the other fuel line is where the return is into the tank i have seen numerous ones that were backwards in which was the problem on the unit and also the adjustment on the needle inside the carb make sure that needle is freely moving if it is not freely moving then bingo there is why it isnt getting fuel
