HELP! Carb leaks fuel in throat when primed

[Tom hadlow]

Hi all I have a carb from a FS91 that when you primed the carb fuel leaks out in to the throat of the carb. How do I go about fixing this. The strimmer will only run on choke when you give it any throttle it dies but I have also found the valve cover gasket to be leaking and will be replacing it any help and advise is much appreciated thanks Tom

 

[stihl #1]

Tom that carb is most likely not repairable. There is an 0-ring on the accelerator pump that if bad, may be leaking fuel, or the high speed nozzle check valve may be the problem. There is also a check valve on the low side and it sounds like one or more of these is at fault. Open the carb and look at the metering chamber. It should be clean and the screw that holds the inlet lever in place should be bright and shiny. If that screw is dull or rusty then the carb is probably done. I have tried tearing them down, using an ultrasonic cleaner and gently flushing the circuits but have never had any success once evidence of contamination is present, indicating corrosion in the circuits and check valve areas that can't be seen or serviced. Just my experience here in the US, and mostly caused by using ethanol fuels and letting the unit sit with fuel in the carb over the off season.
This is an example of the screw I am talking about:

Notice the dull gray color of the aluminum where fuel has contacted it:

As I said, I have never been able to save a carb with contamination, so open yours up and see what it looks like, post a pic and I will look at it for you.
There is a static test you can do before you even take it off the trimmer, but you need a hand operated combination vacuum/pressure pump to do it. From what you have said I know it will fail but if you have the pump I can walk you through the steps.

 

[EFSM]

Hi all I have a carb from a FS91 that when you primed the carb fuel leaks out in to the throat of the carb. How do I go about fixing this. The strimmer will only run on choke when you give it any throttle it dies but I have also found the valve cover gasket to be leaking and will be replacing it any help and advise is much appreciated thanks Tom

If fuel is getting to the throat during priming, the one check valve in the primer base is leaking. It is the one under a welch plug on the underside of the primer base, but unless you have a welch plug that size to replace it with, it is not serviceable. Those Zama carbs are so problematic that as has been stated I would replace it unless you’re an expert on carbs.

 

[SawAddictedFarmer]

These carbs are cheap. Just replace it and run good fuel and you should be good for another 5+ years.

 

[DND 9000]

Don't use pump fuel. Use some type of alcylate fuel like Aspen or something similar and you will not have these problems. Also run it dry for long term storage.