Re-occurring Vacuum Leak STIHL Trimmer

[Merkava_4]

STIHL FS 91 R

It's the middle intake gasket #18 in the IPL below. If I spray carb cleaner at that gasket, the engine will quit.

The other two gaskets #12 and #23 are unaffected by carb cleaner spray.

I've been chasing a problem of the engine shutting off when letting go of the trigger immediately from full throttle.

Letting go of the trigger at part throttle doesn't kill the engine like letting go at full throttle does.

Even with new intake gaskets and the carb mounting nuts very tight, that one center intake gasket always sucks in air.

I have half a mind to double gasket that center gasket unless you guys stop me.

 

[bogieboy]

Tey double gasket, but i would say its more likely a bad oring in the carb accelerator pump that causes the symptoms youre describing... the accelerator pump sits behind the throttle shaft in the carb body, and would be able to draw in carb spray from where you are spraying

 

[EFSM]

Tey double gasket, but i would say its more likely a bad oring in the carb accelerator pump that causes the symptoms youre describing... the accelerator pump sits behind the throttle shaft in the carb body, and would be able to draw in carb spray from where you are spraying

This exactly, unless it is a new carburetor. Sometimes there is enough clearance around the throttle shaft that some carb spray finds its way around and into the intake.

 

[Merkava_4]

Is the o-ring inside the parentheses of Accelerator Pump Kit #41 ?

You guys are brilliant. I would've never thought of that in a million years.

I have one new carburetor left. I guess I'll put it on this morning.

 

[bogieboy]

Is the o-ring inside the parentheses of Accelerator Pump Kit #41 ?

You guys are brilliant. I would've never thought of that in a million years.

I have one new carburetor left. I guess I'll put it on this morning.

View attachment 479601View attachment 479602

Yes, that o-ring is a known failure point on these carbs. I would bet your stalling and low idle issues go away with a new carb, or a rebuild on the accelerator pump circuit.

 

[Merkava_4]

The new carburetor is on, but before I did that, I wanted to set up a control group.

So what I did is double gasket the old carburetor first. Carb cleaner made the RPM drop, but this time the engine didn't die.

Same thing with the new carb. Double gasket the new carb. Carb cleaner would make the RPM drop, but the engine didn't die.

Finally, I decided to try my old tried and true gasket sealant on one gasket only. I use that stuff as a last resort. It's the Permatex 2B sticky brown stuff.

The Permatex is oil and water resistant, but not fuel resistant.

I put a thin layer around the hole of the plastic intake block #20 where it has that ring recess that puts an imprint on the gasket.

I put a thin layer on both sides of the middle gasket #18.

I put a thin layer on the face of the carb where is meets gasket #18.

I should note that the carb face has machine marks on it where the factory surfaced the carb to make it flat. Looks like coarse file marks.

The gasket sealant was able to fill in those machine marks.

Trimmer runs much better. The acceleration is more linear and no more dying after letting go of the trigger at full throttle.

The idle speed is more consistent too.

This is probably a temporary fix because that trimmer is very cantankerous.

bogieboy and EFSM,

ThankYou both very much for your expert advice! I would have never known about that accelerator pump o-ring.

 

[EFSM]

The new carburetor is on, but before I did that, I wanted to set up a control group.

So what I did is double gasket the old carburetor first. Carb cleaner made the RPM drop, but this time the engine didn't die.

Same thing with the new carb. Double gasket the new carb. Carb cleaner would make the RPM drop, but the engine didn't die.

Finally, I decided to try my old tried and true gasket sealant on one gasket only. I use that stuff as a last resort. It's the Permatex 2B sticky brown stuff.

The Permatex is oil and water resistant, but not fuel resistant.

I put a thin layer around the hole of the plastic intake block #20 where it has that ring recess that puts an imprint on the gasket.

I put a thin layer on both sides of the middle gasket #18.

I put a thin layer on the face of the carb where is meets gasket #18.

I should note that the carb face has machine marks on it where the factory surfaced the carb to make it flat. Looks like coarse file marks.

The gasket sealant was able to fill in those machine marks.

Trimmer runs much better. The acceleration is more linear and no more dying after letting go of the trigger at full throttle.

The idle speed is more consistent too.

This is probably a temporary fix because that trimmer is very cantankerous.

bogieboy and EFSM,

ThankYou both very much for your expert advice! I would have never known about that accelerator pump o-ring.

Was the last attempt with new or old carb?