[NA]Yard trimmer fuel system[NA]

Let'sgoflying!

Let'sgoflying!

Touchdown! Greaser!
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Dave Taylor
I hate being flummoxed by patently simple things!
Mine is a Stihl FS90R, but the carbs are used on many different applications and are very common.
I am trying to troubleshoot:
- primer bulb not filling
-excessively lean high speed mixture (despite maxed out adjustment screw)

I think I need to learn how the fuel is supposed to flow from tank to venturi and how the primer bulb gets its fuel, what route it takes to the carb so I can properly trouble shoot.

Before sending ideas:
-this is a new carb
-lots of fuel in tank and nothing is leaking.
-new floppy fuel filter in tank.
-all lines removed, tested for blockage and pressurized underwater for leakage
-everything is pristine clean

When I press the primer bulb, little fuel enters. I am wondering what the thing is on top of the fuel tank with a hose coming out of it, going to the air filter housing. Seems like a clear dime-size membrane or valve with a hole in the center. Could it be involved?
 
Dunno about the system on that particular device but the last time I ran out of gas with a yard tool the tank was 1/2 full. Yup, the tank vent was plugged.
 
Yeah, the one thing to check is that the fuel cap is actually letting air in. Twice I've had to buy new fuel caps (I wasn't able to replace just the gasket) or entire new tanks (when the tubing going to that floppy fuel filter finally gave up the ghost) to get things to work.

Are you talking about the second line to the carb? Most of these two strokes have a return line.
 
This may be a stupid question, but that primer bulb is not like the primer on an AC. It primes the fuel circuit in the trimmer removing air, it doesn't add fuel to the cylinder. You need to repeatedly pump that bulb until it is full of fuel. Mine takes at least three or four full pumps. Are you pumping it enough?
 
Is the in tank fuel filter clogged? Mine had an in tank filter. I just replaced it and the hose that was rotted out.
 
I've got a few FS-90s. Start with the simplest solution first. Start with the vent and the lines. If that's not it you may have two distinct problems (which you probably do). I'd start by checking the primer bulb for a pin hole. A new one is less than $2. Depending on when you bought it, the FS-90R had a recall on the caps a few years ago because the original ones weren't venting.
 
I have a "primer" on a snow-blower that is a rubber "button" that one pushes. it developed a crack over time; it was easily replaced. With a crack, pushing on it didn't move the fuel but just let the pressure out.
 
On the string trimmer it's pretty easy usually. The bulb and the fuel return line usually are transparent. You pump the thing until you see fuel in both.
 
DFH65 said:
Is the in tank fuel filter clogged? Mine had an in tank filter. I just replaced it and the hose that was rotted out.
Click to expand...
"Before sending ideas:
-all lines removed, tested for blockage and pressurized underwater for leakage
-new floppy fuel filter in tank."
 
flyingron said:
On the string trimmer it's pretty easy usually. The bulb and the fuel return line usually are transparent. You pump the thing until you see fuel in both.
Click to expand...
Mine are opaque and black. Might be a good idea to swap for clear so I can see what is happening.
 
Cap'n Jack said:
I have a "primer" on a snow-blower that is a rubber "button" that one pushes. it developed a crack over time; it was easily replaced. With a crack, pushing on it didn't move the fuel but just let the pressure out.
Click to expand...
This new carb came with two new bulbs, there is no leakage but thanks.
 
PaulS said:
This may be a stupid question, but that primer bulb is not like the primer on an AC. It primes the fuel circuit in the trimmer removing air, it doesn't add fuel to the cylinder. You need to repeatedly pump that bulb until it is full of fuel. Mine takes at least three or four full pumps. Are you pumping it enough?
Click to expand...
When the eater was new, it would take 2 pumps to fill it. Now it won't fill at all; keep the ideas coming, we will figure this out. Thank you
 
The actual fuel pump is a diaphragm in the carb that uses engine vibration to pump once the thing is running. The primer bulb just gets fuel into a position to start combustion. I usually prime my Stihl stuff 15-20 quick pulses.
Alcohol in the fuel wreaks havoc on those diaphragms. Could also be a clogged filter (its in the tank at the end of the fuel line, cracked fuel line, clogged return line, or clogged vent.
 
Let'sgoflying! said:
When the eater was new, it would take 2 pumps to fill it. Now it won't fill at all; keep the ideas coming, we will figure this out. Thank you
Click to expand...

Obviously something is wrong somewhere, you could have a defective carb. More likely you have a leak somewhere. Have you tried pumping the bulb until it is full? I'd pump it 15 or 20 times. When I pump my bulb, lol, anyway, there is pressure, when you depress and the bulb doesn't instantly refill as it sucks in gas, does this happen for you ? Most like the primer circuit has a check valve. This could be defective or gummed up. Squirt a little carb cleaner through it and pump it vigorously.

Realistically if the carb is new, I'd bring it back and get another.
 
Let'sgoflying! said:
When the eater was new, it would take 2 pumps to fill it. Now it won't fill at all; keep the ideas coming, we will figure this out. Thank you
Click to expand...
That's a clue right there. It's in the fuel system - something is sucking air. If not the bulbs self, maybe the mount for the bulb. If you've got an extra bulb then try replacing it. I'd swap out the fuel lines, they're cheap. And it's possible the new filter is bad (I've seen it on mine). Tank vent, too.

Any chance you got the lines reversed on the carb? Or the bulb? Sometimes there is some form of check valve (unlikely but possible).
 
a pic.
The line that connects to the filter in the fuel tank goes to the carb body.
The line to the air filter housing, to the valve/membrane thing at the top of the fuel tank.
The line to the primer region comes from or goes to, the top of the fuel tank.
The bulb is filling now.


IMG_5287.JPG
 
It does not run better when the fuel tank cap is removed, so I don't think it's a plugged cap vent.
It does not run better with the fuel filter removed, so I don't think it's causing a problem.
It does run well now, when I press the primer so I feel like it is indeed being starved for fuel.
I am going to change out the fuel lines for new, clear ones tomorrow.
 
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