STIHL trimmer dies when letting go of the trigger

[Merkava_4]

FS 91 R 4-Mix.

Run perfectly fine and runs strong. Let go of the trigger and it dies. Starts right back up on the first pull and idles fine.

Put the trimmer back to work and it dies when letting go of the trigger. Rinse and repeat. I'm at a total loss as to why.

 

[EFSM]

Sounds like it could be too rich and floods out during deceleration. Has anyone ever adjusted the carburetor?

 

[hacskaroly]

I wonder if the low jet has a partial clog? Might be worth cleaning out the carb. Worth checking the spark arrestor too to see if it needs cleaning. Reminds me that I need to service the carb on my FS 91.

 

[Coupe]

Long as it is running well under power it sounds to me that you may
have to tweak the idling screw up a bit till it keeps running when you
let the trigger go. If that does not fix it then it may be your jets.
If you have not played with them before be careful you don't get
it to lean or you may cook your motor.

 

[Merkava_4]

Sounds like it could be too rich and floods out during deceleration. Has anyone ever adjusted the carburetor?

That's exactly what's happening. But if I lean out the high side so that it doesn't die on deceleration, then it's not rich enough to get to full throttle under load. I've been running the high side rich so that I can go max speed at full throttle in wet tall grass. It might be time for a new carburetor.

 

[EFSM]

That's exactly what's happening. But if I lean out the high side so that it doesn't die on deceleration, then it's not rich enough to get to full throttle under load. I've been running the high side rich so that I can go max speed at full throttle in wet tall grass. It might be time for a new carburetor.

Sounds like a rich low side. I should have specified. The 4 mix at idle need set just rich of peak lean; they shouldn’t be as rich at idle as a standard 2 stroke.

 

[lehman live edge slab]

Sounds like a rich low side. I should have specified. The 4 mix at idle need set just rich of peak lean; they shouldn’t be as rich at idle as a standard 2 stroke.

Don’t a lot of the 4-mix have accelerator pumps on the carbs also? Could that be messing up some?

 

[EFSM]

Don’t a lot of the 4-mix have accelerator pumps on the carbs also? Could that be messing up some?

All of them do, at least the OEM carbs. If those o-rings are bad they can cause just about any problem you can think of. Normally it shows up as #1 pulling air through the primer, #2 the idle leaning out after running a few seconds (air is pulled around the bad o-ring and back into the metering chamber), and #3 sluggish acceleration.

 

[Merkava_4]

Sounds like a rich low side. I should have specified. The 4 mix at idle need set just rich of peak lean; they shouldn’t be as rich at idle as a standard 2 stroke.

I swapped out the carburetor. The new one runs perfect at the bench setting. 1-1/4 out on the L and 3-1/2 out on the H.

I'm surprised it runs so well at the bench setting but I'll take it.

Thanks for all your help EFSM. You're one smart cookie.