Stihl 026-help

[thattaway]

This is the first chain saw to beat me down. Cranks on about the third pull. Runs fine at idle, when full throttled it will race to full RPM for a quarter of a second then stall and race to full RPM again, and again. A pulsing type thing. If I creep the trigger slowly it will build to almost half throttle then begin pulsing/racing again. This is a new one for me.

The saw was lightly used by previous owner but sat idle for some years before I got it. When I got it a couple months back I cleaned and rebuilt the carb, installed new fuel line and filter, impulse line, spark plug, and cleaned filter. Adjusted the carb and it ran great. It then sat unused for a month or so. Put a new chain on a week ago and did some cutting. Within minutes it lost power at full throttle while cutting. Brought it home, checked the carb found a bit of trash in the carb screen. Flushed carb completely, reassembled and that's when the pulsing/racing began.

I've torn it apart multiple times since, checked intake boot (no holes or cracks), rechecked all lines but no luck. Have new OEM carb kit ordered and coming (suspect after market kit could have stiff diaphragm but it feels fine) along with an aftermarket carb and a new full round of hose lines and filters. Crank bearings are tight with 0 play and seals look and feel tight as well. Muffler and port are clean. It's almost like it's losing ignition for a split second then firing back off. Fuel line is not pinched. Carb always has gas in reservoir when I break it down. Same gas can used for 3 other stihls and no issues, today even.

What am I missing? Any help appreciated.

 

[RandyT]

If it was a newer saw with the variable ignition timing and internal rev limiter I would suspect faulty ignition module.

 

[OzKioti]

On a previous chainsaw, an air leak in the crankcase caused all sorts of problems, and it went in for warranty repairs three times in quick succession, all to no avail. I got a refund (had to fight for it, persuading the box store that it really was a lemon) and replaced it with a Stihl 026. I can still remember the Stihl salesman saying "Ahhh, you're foolin' with Poulan"!

Still going strong nearly 20 years later.

thattaway, you seem to have covered pretty well all bases, but might be worth checking the crankcase airtightness, which 2 strokes absolutely need.

 

[dodge man]

I seem to be good a diagnosing spark plug problems on Stihl saws lately. If you have the old one pop it in and try it, if not try a new one.

 

[Roadworthy]

If the saw sat with fuel in the carburetor you could have blockage in a tiny passage you're overlooking. It may take a new carburetor instead of just a kit. As stated it could also be the seal on the crankcase has gone leaky. The saw ends up sucking too much air. The magneto or ignition module is another possibility.

 

[thattaway]

I plugged the muffler port, intake port and impulse tube then sprayed soapy water on the crank seals while I turned the crank. Tight as a drum and no bubbles. Found the wires to the kill switch had some nicks so I individually heat shrink covered both. Reassembled the saw with all the new lines, rebuilt carb etc. and it still pulsed/raced. The only part not fooled with was the ignition coil so I ordered a new one off amazon. Slapped it in when it got here and it runs like new. My theory is either the ignition coil was faulty or the nicked wires (even though taped and heat shrink covered) were grounding to each other or via the carb frame. Either way, she is fixed and shouldn't need any new parts for a while. Thanks for all the suggestions.

 

[dodge man]

Thanks for the feedback, glad you got it going.