How does the primer bulb on a string trimmer pump gas?

Y

Yardguy

Member
Joined
Aug 13, 2012
Threads
7
Messages
37
  • / How does the primer bulb on a string trimmer pump gas?
I have a Stihl FS55 string trimmer. Engine would start but wouldn't keep running so I disassembled and cleaned the carburetor using carburetor cleaner in a rattle can. Reassembled and installed and found the primer bulb won't fill with gas. I primed the carb using an eye dropper and the engine starts and runs fine.

Disassembled carb again. Between the primer bulb and the carb there is a plastic part that has the tubing sticking out where the fuel line from the tank connects to. The primer bulb is supposed to suck gas from the tank through that tubing. When I try to blow or suck on the tubing it seems blocked. I assume that I should be able to blow air into the tubing (like gas going into the primer bulb) but not be able to suck air out of the tubing. Is that right?

Thanks,
Yardguy
 
M

mechanic mark

Lawn Pro
Joined
Jul 15, 2013
Threads
175
Messages
7,412
  • / How does the primer bulb on a string trimmer pump gas?
Stihl FS55 Parts Replace spark plug, fuel & air filters with Stihl OEM parts. Use high octane gas with Stihl 2 cycle mix, also check fuel lines for being hard and brittle and if so replace them. I would also pick up Stihl fuel treatment and mix per directions, it does do what the bottle says relating to ethanol gas. Let us know how it goes, thanks.
 
B

bertsmobile1

Lawn Royalty
Joined
Nov 29, 2014
Threads
64
Messages
24,705
  • / How does the primer bulb on a string trimmer pump gas?
The primer bulb sucks air from the carb and sends it to the petrol tank.
It's job is not to pump gas but to purge the system of air.

Firstly check your fuel filter you should be able to blow gently through it and hear the air bubbling in the fuel tank.
Next make sure you have the hoses around the right way.
The one nearest the primer bulb is the fuel return and should go to the tube that is just inside the fuel tank.
the one that is furthest away is the fuel in and should be connected to the fuel line with the filter on it.

the primer has a little one way valve in it ( which is replaceable ).
So when you press in on the bulb the air is forced down past the valve into the petrol tank.
When it pops back out it sucks the air in the fuel chambers in the carb into the primer which draws fuel out of the tank into the carb.
Continuing the process will eventually expell all of the air and the bulb will fill with fuel.
Very importsnt is the heavy walled tube must be connected between the carb & fuel filter as thin walled tube can collapse & block off the fuel supply.
Usually the input line will be bigger overall than the return line.
 
Y

Yardguy

Member
Joined
Aug 13, 2012
Threads
7
Messages
37
  • / How does the primer bulb on a string trimmer pump gas?
bertsmobile1 said:
The primer bulb sucks air from the carb and sends it to the petrol tank.
It's job is not to pump gas but to purge the system of air.

Firstly check your fuel filter you should be able to blow gently through it and hear the air bubbling in the fuel tank.
Next make sure you have the hoses around the right way.
The one nearest the primer bulb is the fuel return and should go to the tube that is just inside the fuel tank.
the one that is furthest away is the fuel in and should be connected to the fuel line with the filter on it.

the primer has a little one way valve in it ( which is replaceable ).
So when you press in on the bulb the air is forced down past the valve into the petrol tank.
When it pops back out it sucks the air in the fuel chambers in the carb into the primer which draws fuel out of the tank into the carb.
Continuing the process will eventually expell all of the air and the bulb will fill with fuel.
Very importsnt is the heavy walled tube must be connected between the carb & fuel filter as thin walled tube can collapse & block off the fuel supply.
Usually the input line will be bigger overall than the return line.
Click to expand...
Thank you very much for your explanation!

>>the primer has a little one way valve in it ( which is replaceable )<<
Oh, is the one way valve under the round metal piece that's in the plastic base for the primer bulb? That round metal piece is about 1/4" in diameter and had plastic wrap like film sticking out around its perimeter.

To verify that I'm understanding the prime operation:
When the bulb is pressed it forces air* into the tank via the "tubing on the plastic base for the primer bulb" that has a fuel line going into the tank. When the bulb is released, it sucks fuel via the other fuel line into the "tubing on the carburetor body" and that fuel ends up in the bulb. When the bulb fills with fuel, pressing it forces fuel into the air intake of the carburetor and also some fuel is forced back into the tank instead of air*. Is this correct?

Thanks,
Yardguy
 
B

bertsmobile1

Lawn Royalty
Joined
Nov 29, 2014
Threads
64
Messages
24,705
  • / How does the primer bulb on a string trimmer pump gas?
Yardguy said:
Thank you very much for your explanation!

>>the primer has a little one way valve in it ( which is replaceable )<<
Oh, is the one way valve under the round metal piece that's in the plastic base for the primer bulb? That round metal piece is about 1/4" in diameter and had plastic wrap like film sticking out around its perimeter.

To verify that I'm understanding the prime operation:
When the bulb is pressed it forces air* into the tank via the "tubing on the plastic base for the primer bulb" that has a fuel line going into the tank. When the bulb is released, it sucks fuel via the other fuel line into the "tubing on the carburetor body" and that fuel ends up in the bulb. When the bulb fills with fuel, pressing it forces fuel into the air intake of the carburetor and also some fuel is forced back into the tank instead of air*. Is this correct?

Thanks,
Yardguy
Click to expand...

Going to depend upon exactly which carb you have.
The replaceable valve is generally a pinky orange colour and lives under the bulb.
Other thing to rememer is the whole box & dice has to be airtight or it will suck in air at all the gasket faces.
Usually we pop them in a glass jar of water and blow nto them looking for air bubbles leaking out.

However it sounds more like you have the fuel lines back to front,
Remember the line nearest the primer ( in the black plastic bit ) is the return line, the other one is the fuel supply.
 
Y

Yardguy

Member
Joined
Aug 13, 2012
Threads
7
Messages
37
  • / How does the primer bulb on a string trimmer pump gas?
bertsmobile1 said:
Going to depend upon exactly which carb you have.
The replaceable valve is generally a pinky orange colour and lives under the bulb.
Other thing to rememer is the whole box & dice has to be airtight or it will suck in air at all the gasket faces.
Usually we pop them in a glass jar of water and blow nto them looking for air bubbles leaking out.

However it sounds more like you have the fuel lines back to front,
Remember the line nearest the primer ( in the black plastic bit ) is the return line, the other one is the fuel supply.
Click to expand...
Since the engine runs fine after I prime it with an eye dropper, I'm quite sure that the lines are connected correctly. Also one line is longer to reach the upper tubing so that helped identify which line goes where.

If I can't get the primer bulb to work, I'll just squirt some fuel into the carb using an eye dropper when I first start the engine. It's not that hard to pop open the cover and remove the air filter to do that.

Thanks much for you help,
Yardguy
 
You must log in or register to reply here.

Footer Links 1

Top