MS 260 puzzler

[pooroldcraig]

So, last year I was cutting up a sizable oak tree with an that had been down for some time, using an MS260 that has run flawlessly for some time. everything was going fine for a few hours and I had the tree nearly done when I had a sudden increase in rpm, to the tune of a couple thousand rpms (best guess). Kind of flipped me out and I shut the saw off and called it a day. The following day i started it right up, it seemed to run fine and I shrugged and proceeded to cut. A few minutes later, same scenario, only this time the saw did not want to restart. I shelved it and went on about my business until a few weeks ago. I did a pressure/leak test and the motor is airtight. I cleaned the carburetor (didn't seem to need it, really) and checked to make sure I hadn't sheared flywheel key and made sure the fuel filter was flowing properly. Compression was at 165, if I remember correctly. I did a muffler mod to it and put the saw back together with fresh fuel and went to tune it, only to find out I'd run the high speed needle in crooked and stripped out the threads in the carb. Put on an aftermarket carburetor that appears to be identical to the one that came off of it and proceeded to try again. Saw started up quickly and I warmed it up with a few cuts in some "12 oak and stuck a tach on it. At WOT I again experienced the sudden surge in rpms, jumping from about 12,500 to 14,500+ after a few seconds with only the barest hair of a turn to the high end screw. Repeated attempts had the same result. Running rich to runaway rpms in a fraction of a second with no further adjustment to the mixture screws. What, besides a clogged tank vent might cause this? Any suggestions on what else I should check? It seemed like a lean fuel thing and I'm guessing it wasn't carb issue, as the symptom was the same for two different units. I'd richened both screws up by half a turn over factory settings, by the way, and I certainly had my "burble". What am I missing?

 

[lone wolf]

So, last year I was cutting up a sizable oak tree with an that had been down for some time, using an MS260 that has run flawlessly for some time. everything was going fine for a few hours and I had the tree nearly done when I had a sudden increase in rpm, to the tune of a couple thousand rpms (best guess). Kind of flipped me out and I shut the saw off and called it a day. The following day i started it right up, it seemed to run fine and I shrugged and proceeded to cut. A few minutes later, same scenario, only this time the saw did not want to restart. I shelved it and went on about my business until a few weeks ago. I did a pressure/leak test and the motor is airtight. I cleaned the carburetor (didn't seem to need it, really) and checked to make sure I hadn't sheared flywheel key and made sure the fuel filter was flowing properly. Compression was at 165, if I remember correctly. I did a muffler mod to it and put the saw back together with fresh fuel and went to tune it, only to find out I'd run the high speed needle in crooked and stripped out the threads in the carb. Put on an aftermarket carburetor that appears to be identical to the one that came off of it and proceeded to try again. Saw started up quickly and I warmed it up with a few cuts in some "12 oak and stuck a tach on it. At WOT I again experienced the sudden surge in rpms, jumping from about 12,500 to 14,500+ after a few seconds with only the barest hair of a turn to the high end screw. Repeated attempts had the same result. Running rich to runaway rpms in a fraction of a second with no further adjustment to the mixture screws. What, besides a clogged tank vent might cause this? Any suggestions on what else I should check? It seemed like a lean fuel thing and I'm guessing it wasn't carb issue, as the symptom was the same for two different units. I'd richened both screws up by half a turn over factory settings, by the way, and I certainly had my "burble". What am I missing?

Vac test it again and wiggle the crank ends to be sure there is no leak maybe the bearings are loose.

 

[Adirondackstihl]

1/2 turn is "huge"

 

[pooroldcraig]

I opened the muffler up a good bit. Two extra holes the size of the stock opening. I also wasn't sure about the aftermarket carb setting vs stock and figured better safe than sorry for a starting point.

 

[pioneerguy600]

Either test the fuel line with a vac tool or replace the fuel line, the 026 and 260 saws are notorious for a cracked fuel line right after the grommet where they exit the tank, a visual inspection is often not good enough to detect this crack in the line.

 

[kyle.kipple]

Either test the fuel line with a vac tool or replace the fuel line, the 026 and 260 saws are notorious for a cracked fuel line right after the grommet where they exit the tank, a visual inspection is often not good enough to detect this crack in the line.

+1

Since your digging into it, I would change the impulse and fuel lines regardless.
Also check out the intake vigorously.


Sent from my iPhone using Tapatalk

 

[mountainlake]

I just had a saw do the same, run good for a few minutes then lean out, it was a cracked fuel line, Steve

 

[XSKIER]

Cracked intake boot flexing with handles?