FS 350 brushcutter runs fine for a short period, sputters and stalls.

[hotgemini]

First up, a thanks to the regulars here on AS. Reading back through a few dozen threads played a big part in convincing the M-I-L to buy the F-I-L a pro grade MS260 rather than a homeowner spec saw (MS250 etc) and he's over the moon with his new toy.

Now to my problem, I've got an FS 350 stihl brushcutter and generally it's a fantastic thing but of late it's developed an intermittent problem. It'll be cutting fine for 5 to 50 minutes but will then lose power, sputter and drop basically back to idle revs and usually then die. If I back off the throttle it'll idle for a bit and then I can give it a short rev but it pretty quickly dies again.

Now for my money it felt like something starving for fuel, so I popped out the fuel strainer/inlet, it was covered in muck and clearly past it's best so I replaced it and put a new spark plug and air cleaner element in at the same time.

Unfortunately the problem hasn't gone away and if anything the length of time it works before the problem starts is getting progressively shorter on average.

Thoughts? The green (silicone?) fuel line seemed okay. Garbage in the carburettor perhaps? If so how would I go about fixing it?

On the other hand, a rule of mine is "If it feels like a fuel problem, suspect spark and vice versa" so I guess I wonder if it's a spark problem? Would it be worth throwing a new spark plug lead on it? Do the ignition modules fail and if they do, does it match my symptoms?

It's a 40ish minute round trip two towns down the valley to the nearest stihl shop, so I'm happy to err towards replacing more parts rather than going one item at a time and spending all day driving back and forth.

Thanks in advance!
-Adam.

 

[newsawtooth]

By no means am I a small engine mechanic...but two things to check and then proceed with previous plans. Look at the fuel vent on the gas tank, if the uptake was clogged the vent could be clogged as well. Secondly take a look at the spark arrester. They can be covered in carbon to the point they'll choke out a motor. Best of luck.

 

[hotgemini]

Howdy,

Thanks for the suggestions and don't worry, I'm no small motor mechanic either.

Had previously tried running it with the fuel cap open which by my understanding should rule out a blocked vent (sorry, should have mentioned).

Pulled the muffler off, doesn't appear to run a spark arrestor screen and blowing through the muffler doesn't suggest it's particularly blocked up, but thanks for the suggestions anyway, eventually I'll pin it down.

Any other thoughts?

-Adam.

 

[hotgemini]

Don't worry, I'm no small engine mechanic either.

Had already tried running it with the fuel cap off, which by my understanding rules out a vent issue. (sorry, should have mentioned)

Pulled the muffler off today and had a look, doesn't appear to run a spark arrestor and blowing through the muffler didn't appear to reveal any particular restriction.

Thanks for the thoughts though, sooner or later we'll pin it down and get this thing singing away again soon.

Bloody ripper of a brushcutter when it was running sweet, nothing stood in its way. Makes me curious as to what the MS450/550 must be like!

So, any other thoughts/ideas?

-Adam.

 

[hotgemini]

Hmm, seems the reply I thought went missing turned up in the end, oh well, guess if it was worth saying, it was worth saying twice.

Anyone got any more ideas, back in the big smoke now for a while but would like to have a selection of parts/ideas to try when I head out to the farm next.

-Adam.

 

[ted4bp]

How long have you had the unit? Have you tried a new fuel filter or rebuilding the carb?

 

[hotgemini]

I've had it for about 2 years, but it came to me 2nd hand so I don't know it's history.

Put a new fuel filter/pick-up thing on it, along with a new air filter and spark plug.

Haven't tried rebuilding the carb. How involved (I've rebuilt half a dozen automotive carburettors so I don't imagine I'll be out of my depth) is it? What parts are needed (gaskets etc)?

 

[Saw Bones]

Howdy,

Thanks for the suggestions and don't worry, I'm no small motor mechanic either.

Had previously tried running it with the fuel cap open which by my understanding should rule out a blocked vent (sorry, should have mentioned).

Pulled the muffler off, doesn't appear to run a spark arrestor screen and blowing through the muffler doesn't suggest it's particularly blocked up, but thanks for the suggestions anyway, eventually I'll pin it down.

Any other thoughts?

-Adam.

I had a similar problem with my FS 90. I took the carburator apart and found pieces of what appeared to be orange silicone gasket material in the pulse line from the engine to the carb, and in the carburator itself. I cleanded that out and had no further problem. No clue how that stuff got in there.

Another thought is possibly weak spark caused by a coil that is slowly failing. (Just an off the wall guess) Coils will sometimes cause problems as they heat up and get to a point where they won't run at all. Check your spark while it is running good, and compare it to what you see when it is not.

 

[sawinredneck]

I'll second the coil suggestion. Next time it dies and doesn't start check for spark. Let it sit for a while then check for spark again, it will be easy to tell.
But a unit that old, and used that heavily, a coil wouldn't be far out of reach. They run fine until they heat up, crap out, cool off and run fine again, but the run times get shorter and shorter as the coil breaks down.

 

[hotgemini]

Just a follow-up on this. Eventually gave in and left it with with a local stihl dealership who even market themselves as 'the brushcutter specialists'. The initial diagnosis was that it would be the ignition module and they'd probably have it in stock and it'd be ready that afternoon....

To cut a long story short, 7 weeks later we got the brush cutter back having had both the carb rebuilt and the ignition module replaced. The problem is gone, so thats a win, but I'm still none the wiser as to the actual cause of the issue.

Seems a little down on power compared to before the issues started, tune might need a bit of a tweak.