# sthil fs 45 wont prime



## awkgreen (Jul 10, 2011)

I have stipped and cleaned the carb,cleaned the tank and filter,inspected fuel pipes for leaks,and the primer bulb still does'n't draw fuel from the tank


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## FATGUY (Jul 10, 2011)

anytime I've run into that, there's crap in the fuel inlet screen opposite the metering needle. If not, maybe bulb itself is cracked....


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## Stihldoc (Jul 10, 2011)

4140 series trimmers are one of the most dependable little trimmers being made today. I love them. That said, they have one quirk that causes more headaches than all others combined. I'll get to the solution to your problem, but first a little info on why it happened. Gas thats stored more than 60 days loses octane, turns into varnish, and in some cases- becomes acidic. A no purge situation needs some exploration. Does the bulb empty and not refill (often stays compressed), or does it pop back up but not fill with fuel. In the former case, where the bulb doesn't pop up, the inlet screen in the fuel pump side of the carburetor is usually blocked. It is blocked by debris that is DISSOLVED in the fuel. It made it past the fuel filter so it must be fine stuff. Its like sugar dissolved in iced tea- you know its there but you can't see it because it is in solution. What kind of debris you ask? Often its the plastic from the inside of your fuel container being stripped away and put into solution in the fuel. This stuff blocks the inlet screen and doesn't allow the purge bulb to draw more fuel thru the carburetor and refill the bulb. For starters, clean or replace the inlet screen. If the purge bulb pops back up but doesn't draw fuel, chances are that one or both of the check bulbs in the purge base are stuck open or have failed altogether. When I strip one of these Zama carbs down, I look for any corrosion on the fuel pump cover and if its present, you'll NEVER get the carb clean enough to be dependable. I use an Omegasonics ultrasonic cleaner and you can get the loose crap off, but the zinc carb will continue to corrode and block the screen and the main fixed jet. Most of the issues with 4140 carbs are with the fully adjustable (high & low mixture screws). The older BIS (balanced idle system) carbs with only one air bleed screw don't seem to be as problematic. If you have a fully adjustable carb and you can truly clean it, install a Zama RB-100 kit and do the mixture adjustments. If you have a faulty purge base, you can replace it and install a kit, and spend almost as much as a new carb costs. When I open up one of these carbs and see corrosion, I don't bother to clean it, I just replace it. I know that this will generate the usual "parts changer, not technician" comments, but when the new carb is $32.00 and it takes a few minutes to install it along with a new fuel filter, why should I waste time at $76.00 per hour to overhaul it? It is cheaper (parts and labor) to replace the carb than try to repair. If you have a fully adjustable carb (2 mixture screws & an idle speed screw), order & install part #4140-120-0619. This is the latest version of the carb and usually needs little or no adjustment after installation. Be sure to replace the fuel filter #0000-350-3506 too. Then, use fresh mix and don't keep it for more than 60 days. Always drain the fuel for off-season storage. I love these dependable little trimmers and almost all fuel issues are directly related to improper fuel storage. Stihldoc replaced 4 of these carbs yesterday. I like these trimmers- people just have to treat them correctly to get dependable service out of them for many years to come.


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## hotshot (Jul 11, 2011)

*Bulb is really a purger, not a primer...*

Sounds like the lines are simply installed backwards.

The bulb dumps out to the tank in the pushed stroke, & then sucks from the carburetor metering side 
in the return/refill stroke via check valve; in other words the barb mared "IN" goes to the carb, and "OUT" 
goes to the fuel tank.


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## awkgreen (Jul 11, 2011)

*primer bulb(purger) will not fill up*

I'm pretty sure that the pipes are on correctly as I marked them before I took them off.There are no holes in the bulb either. I dangled the carb in a jar of fuel, and when I press the bulb I can hear it ,but it just won't fill.


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## hotshot (Jul 11, 2011)

*Great!*

Well you're fine then, if the purge bulb tests out correctly for flow and checking.

The only problem to draw fuel in could be a restriction in the fuel filter, the fuel pump diaphragm, the tiny screen in the
inlet needle port, or inlet needle valve/seat assembly. 

What was the pop off pressure when you pressure tested the carb without the metering cover on it?


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## awkgreen (Jul 11, 2011)

*carb not purging*

I have stripped the carb down and cleaned it in a ultrasonic tank.I have done the same with the black plastic block,and I have blown the tank filter out with compressed air. The plug filter in the carb is shiny clean, I can't pressure test as I don't have the equipment......I really apreciate all your suggestions please keep em coming..... what now?


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## Bsox67 (Apr 30, 2014)

Stihldoc said:


> 4140 series trimmers are one of the most dependable little trimmers being made today. I love them. That said, they have one quirk that causes more headaches than all others combined. I'll get to the solution to your problem, but first a little info on why it happened. Gas thats stored more than 60 days loses octane, turns into varnish, and in some cases- becomes acidic. A no purge situation needs some exploration. Does the bulb empty and not refill (often stays compressed), or does it pop back up but not fill with fuel. In the former case, where the bulb doesn't pop up, the inlet screen in the fuel pump side of the carburetor is usually blocked. It is blocked by debris that is DISSOLVED in the fuel. It made it past the fuel filter so it must be fine stuff. Its like sugar dissolved in iced tea- you know its there but you can't see it because it is in solution. What kind of debris you ask? Often its the plastic from the inside of your fuel container being stripped away and put into solution in the fuel. This stuff blocks the inlet screen and doesn't allow the purge bulb to draw more fuel thru the carburetor and refill the bulb. For starters, clean or replace the inlet screen. If the purge bulb pops back up but doesn't draw fuel, chances are that one or both of the check bulbs in the purge base are stuck open or have failed altogether. When I strip one of these Zama carbs down, I look for any corrosion on the fuel pump cover and if its present, you'll NEVER get the carb clean enough to be dependable. I use an Omegasonics ultrasonic cleaner and you can get the loose crap off, but the zinc carb will continue to corrode and block the screen and the main fixed jet. Most of the issues with 4140 carbs are with the fully adjustable (high & low mixture screws). The older BIS (balanced idle system) carbs with only one air bleed screw don't seem to be as problematic. If you have a fully adjustable carb and you can truly clean it, install a Zama RB-100 kit and do the mixture adjustments. If you have a faulty purge base, you can replace it and install a kit, and spend almost as much as a new carb costs. When I open up one of these carbs and see corrosion, I don't bother to clean it, I just replace it. I know that this will generate the usual "parts changer, not technician" comments, but when the new carb is $32.00 and it takes a few minutes to install it along with a new fuel filter, why should I waste time at $76.00 per hour to overhaul it? It is cheaper (parts and labor) to replace the carb than try to repair. If you have a fully adjustable carb (2 mixture screws & an idle speed screw), order & install part #4140-120-0619. This is the latest version of the carb and usually needs little or no adjustment after installation. Be sure to replace the fuel filter #0000-350-3506 too. Then, use fresh mix and don't keep it for more than 60 days. Always drain the fuel for off-season storage. I love these dependable little trimmers and almost all fuel issues are directly related to improper fuel storage. Stihldoc replaced 4 of these carbs yesterday. I like these trimmers- people just have to treat them correctly to get dependable service out of them for many years to come.


THAT was a response! I am also having trouble with my primer bulb on the FS 45 Stihl trimmer. The bulb has fuel in it but will not pop back up. I'm gathering from your well described post that a full carb kit replacement might be in order? The unit is only 3 yrs old but was stored with old gas. If I replace the screen i may as replace everything...Correct? And thank you for spending the time helping out the less knowledgeable, but eager to learn people such as myself.


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## Wood Doctor (Apr 30, 2014)

Hey AwkGreen, see my recent "dirt cheap carb fix" thread and try the 24-hour mixed fuel soak routine for that carb and the fuel filter in an empty peanut butter jar. You really have nothing to lose. However, in my case, the primer was pulling but the carb refused to take over from there.

BTW, SthlDoc's post is a classic. I've captured and filed it. I only wish I was making $76/hr fixing these machines.


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