Stihl FS38 trimmer primer bulb sucking air in and runs badly

[XGamer223]

Hello everyone,
so a FS38 trimmer (looks fairly new) has some issues.

First off, the primer bulb sucks air in every time you let off the bulb, lots of air bubbles to be seen. Also, the bulb doesn't fill more than like 50% or less no matter how often you press it.

Secondly, the trimmer runs badly overall. It doesn't respond to throttle properly, it starts bogging down every little more throttle you add to it, too long on the throttle kills the motor completely. It does idle, but not in "harmony", the rpm stay fine until at one point it just drops for half a second, the other second it backs up again, and it repeats like that. Interestingly, it sometimes runs just fine, and the other moment it doesn't, it doesn't matter if it's cold or hot, primed or not, it just decides randomly to run nice or not, but when it does run nice - it doesn't last too long. Also, I noticed that it stops bogging down when I prime the bulb while I give it some gas. Also, I had some time to tune the carburetor properly and made it run really nice, but as I said earlier, it doesn't last long.

Thirdly (not much of an issue, but it's really weird), it runs on choke like it's not on choke, and it doesn't really matter to it much. Also, when I remove the air filter I can't shut it off (?). When I hit the off switch, it stays running for 5-6 seconds until it does itself so (this doesn't happen with the air filter on).

Here's a video of the primer bulb in action:

https://imgur.com/IjbPb8u

What I did:
- replaced metering diaphragm, the gasket under it, the pump diaphragm and the gasket under it
- replaced the primer bulb
- placed a verified working spark plug from a working trimmer
- cleaned air filter
- inspected fuel filter and the fuel lines
- cleaned the carb twice
- placed zip ties on the fuel lines since they seemed stretched

What else could be there?
Thanks anyone in advance!

 

[bertsmobile1]

Pull it off
Put it in a big glass jar with enough water to fully cover it
Put a line on both the intake & return.
Pump up the fuel intake tube no bubbles should come from anywhere except the return line
Pump up the return line, No bubbles should be seen from anywhere.

Fill the tank to the brim of the filler cap
Problem goes away = crack in the fuel line usually just where it goes through the tank

Put you finger over the return outlet then press down the purge bubble.
You should not be able to fully depress it with the hole blocked.
Put your finger over the inlet lightly then depress the purge bubble
You should feel some sucking
With the bubble fully down shove your finger hard on the inlet to stop air entering.
The bubble should stay down till you pull your finger off .

 

[StarTech]

One problem Bert, some cube carburetors due to design will always suck air in through the metering chamber; therefore, the primer bulb will not always stay fully depressed with the inlet plugged.

Then of course there can be a missing welch plug, bad main nozzle check valve, bad enrichment pump which this carburetor may or may not have. Just depends the carburetor model number. Without having the carburetor model number it will be hard to know how this one is setup.

This one could have mixtures screw paths that opens directly to metering chamber without check valves so air is always sucked in via the primer action or the mixture chamber might be be missing a welch plug. Really need the carburetor in hand to see the design as even IPLs don't show these things.

Also this 4140 engine may a spark arrester that is partially clogged or the carburetor simply needs tuning or the cylinder could be heavily scored.

Either way working these engines is not wham bang thank you madam. It takes an experience mechanic to repair them at times. Most DIYers just slap another carburetor on it to see if it works or not.

 

[Fish]

Have you tried just richening up the mixture screws? The "primer" looks to be operating OK. It is actually a "purge" and pulls fresh gas through the carb and returns the excess to the tank.
You might just pull the limiter caps and richen up your screws.

 

[Fish]

There is a limiter to prevent richening up the screws very much, so pulling them is what is next to be able to richen them further.

 

[StarTech]

Those images make it look easier than it is. Usually the limiter need to be destroyed to get it off. Using a pair pliers simply don't work as these have locking tabs. Probably why I got a bag full of new limiter caps.

 

[Fish]

I never put any back on.

 

[StarTech]

I haven't been so lucky not to have customers that like to turn screws. To me it is 0.88 well spent not to need to retune an engine every few months or have a needle to get lost.

 

[XGamer223]

Pull it off
Put it in a big glass jar with enough water to fully cover it
Put a line on both the intake & return.
Pump up the fuel intake tube no bubbles should come from anywhere except the return line
Pump up the return line, No bubbles should be seen from anywhere.

Fill the tank to the brim of the filler cap
Problem goes away = crack in the fuel line usually just where it goes through the tank

Put you finger over the return outlet then press down the purge bubble.
You should not be able to fully depress it with the hole blocked.
Put your finger over the inlet lightly then depress the purge bubble
You should feel some sucking
With the bubble fully down shove your finger hard on the inlet to stop air entering.
The bubble should stay down till you pull your finger off .

Hello and thanks for your answer.
I did the tests. This is how it went:

Pull it off
Put it in a big glass jar with enough water to fully cover it
Put a line on both the intake & return.
Pump up the fuel intake tube no bubbles should come from anywhere except the return line
Pump up the return line, No bubbles should be seen from anywhere.

This went good, no bubbles anywhere but the return line when pressing, and no bubbles when letting it off (BUT, there sometimes is a small bubble on the black plastic where the fuel bulb sits in. There's a small (factory) cut in there where a bubble can be seen escaping. But not always, and not sure, I could be mistaken).

Fill the tank to the brim of the filler cap
Problem goes away = crack in the fuel line usually just where it goes through the tank

Did so, but no better.

Put you finger over the return outlet then press down the purge bubble.
You should not be able to fully depress it with the hole blocked.
Put your finger over the inlet lightly then depress the purge bubble
You should feel some sucking
With the bubble fully down shove your finger hard on the inlet to stop air entering.
The bubble should stay down till you pull your finger off .

Holding return line blocked, I can't depress the bulb.
Finger over inlet pipe, yes, I can feel some sucking.
No, the bubble doesn't stay down, it returns normally as if I didn't block the inlet line.

Just as note, I placed a Stihl FS 45 carb on it (same carb) that I can confirm that works fine, and the trimmer runs like new, thus meaning that fuel lines and filter are in good shape, just like the cylinder or spark arrestor, and spark plug too.
The carb is a ZAMA S97.

P.S. sorry for the late answer, was really busy.

 

[XGamer223]

One problem Bert, some cube carburetors due to design will always suck air in through the metering chamber; therefore, the primer bulb will not always stay fully depressed with the inlet plugged.

Then of course there can be a missing welch plug, bad main nozzle check valve, bad enrichment pump which this carburetor may or may not have. Just depends the carburetor model number. Without having the carburetor model number it will be hard to know how this one is setup.

This one could have mixtures screw paths that opens directly to metering chamber without check valves so air is always sucked in via the primer action or the mixture chamber might be be missing a welch plug. Really need the carburetor in hand to see the design as even IPLs don't show these things.

Also this 4140 engine may a spark arrester that is partially clogged or the carburetor simply needs tuning or the cylinder could be heavily scored.

Either way working these engines is not wham bang thank you madam. It takes an experience mechanic to repair them at times. Most DIYers just slap another carburetor on it to see if it works or not.

Thanks for your answer!
The carb is a ZAMA S97 (sorry for not noting before, thought the Stihl model was enough..).

The engine is fine overall, just like the spark plug, fuel lines, filter, spark arrestor etc. since I placed a carb from a different Stihl (FS 45), but it's the same carb. And the engine ran like it's new, 0 issues, even gave it a nice mow and still ran good for 15 minutes straight.

Here are some images (it's an album):

https://imgur.com/a/LWBwUTW

I can upload more, and more detailed if needed.
Sorry for the late answer, was busy.