Stihl 391 Stalling Issue

Arborist Forum

Help Support Arborist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Just bought a used Stihl 391 on Craigslist for $400 w/ 20" bar and new chain. Looked ok, started ok, seller told a pretty good story. Bucked thru 4 or 5 30" dia American Elm logs, ran strong. Then it stalls while cutting. Starts right up on 1st pull, runs about 15 seconds, then stalls (sounds like the fuel system is leaning out). Shot a video, posted to YouTube, https://youtu.be/SKfB7qj-LQI .
Stalls out like clockwork every 15 seconds, starts right up on 1st pull, runs for 15 seconds. It is full of high-dollar pre-mix gas, opened/closed fuel tank to verify it was not a tank venting issue.

Question for the experienced Stihl users, is there any particular issue that the 391 is known for that may be causing this strange stalling issue? ie weak fuel pump desigh, known carburator issues, or even something in the electronic ignition?
I will troubleshoot this saw and post the fix to this forum.
Based on the description and the video... The saw may have had an issue when you bought it, like an air leak or mis-adjusted carburetor running too lean. Cutting bar buried in hardwood with a possibly dull chain finished it off.

Pull the muffler and post photo(s) of the piston thought the exhaust port.
:thisthreadisworthlesswithoutpictures:

Highly unlikely fuel or ignition problem as these don't usually happen fast. Piston seizures do.
 
Had something similar on a 390. Would run a few minutes then stall. Installed a new piston, crank seals, coil, fuel line, and carb rebuild but still stalled after a few minutes. Piddled with that saw for 3 or 4 years off and on. Finally noticed the fuel line made a sharp bend from the top of the tank to the carb inlet. Bent a short 90 from some metal tubing about the size of the inside of the fuel hose, worked it in the line to hold the bend open and the saw has run fine ever since. One of my favorite saws now.
 
Again, many thanks to everyone for sharing your years of experience. Went to work this morning on the new-to-me MS-391 which was stalling after about of 15 seconds of running. Looked at the side plates as best I could without stripping too much plastic stuff off, no signs of leakage or looseness. Looked for the exhaust screen, but appears prev owner has already yanked it out (Pic #1) . Looked at spark plug, appeared perfect.
Drained fuel into gas beaker, no debris or water, color of fuel consistent with the high-dollar ethanol free pre-mix owner said he was using. Pulled fuel filter, no debris on outside. Gave fuel filter the blow/suck test, blew/sucked quite a bit harder than I thought it should while soaked w/ fuel. Blew fuel filter out w/ compressed air, sucked/blew better but not real good. Went & bought a new fuel filter for USD 5.50 (Pic #2). Did not yank the muffler to look at cylinder, compression is good, saw was running early yesterday before going into "stall mode" and was running strong in between stalls yesterday.
Refilled fuel tank w/ fresh fuel. Fired up saw and ran about 1/2 tank of gas bucking some American Elm for firewood, ran perfect, very strong, without the stalling issue, throwing mounds of cuttings. So I guess stall issue is "fixed", fingers crossed, carrying a MS-170 for backup tomorrow!
 

Attachments

  • Pic 1 Exhaust.jpg
    Pic 1 Exhaust.jpg
    682.5 KB · Views: 8
  • Pic 2 New Fuel Filter.jpg
    Pic 2 New Fuel Filter.jpg
    654.9 KB · Views: 8
Looked for the exhaust screen, but appears prev owner has already yanked it out (Pic #1).
Removing the spark screen will cause the engine to lean out. You need to verify that it is not TOO lean and properly tuned after the screen was removed. Also check to see if the limiter caps are still intact.

The rest of this makes no sense... unless the P.O. recently switched fuel types which may have broken some crud loose in the tank which clogged the original filter.
 
I hate how bloody expensive stuff is in Australia. It’s $10 USD for a ****ing filter out here and you guys get them for $5.
tools are so expensive here too, roughly 35% more…

Glad the new filter helped, not surprising, not sure how none of us thought of that LOL.

Simple things first hey..
 
He still hasnt pulled the muffler to check the piston condition.
pretty much a no brainer first thing you do with a new to you saw.
Especially if its showing lean running symptoms.

I guess we will see in due course.

or a new post saying help my new 391 wont start at all.
 

Attachments

  • pre-heater_MS391.jpg
    pre-heater_MS391.jpg
    149 KB · Views: 7
  • heater.jpg
    heater.jpg
    846 KB · Views: 6
Back
Top