Stihl Weedwacker Intermittently Dying

[pen]

I have a Stihl weed wacker (FS40) that I've had for 17 years that has been a storm trooper.

2 years ago this problem started once in a great while and it's steadily gotten more consistent.

What happens is if I'm doing light trimming and am on and off the throttle, eventually it will be idling along as I'm moving to a new spot and when I hit the gas it doesn't want to rev up. If I hold the throttle wide open it will just die. Pull the cord and it'll usually come back to idle just fine. If I feather the throttle sometimes I can get it up to full speed again and it will run there for as long as I want, so long as I don't let off the throttle or else it will settle back to idle, run fine, but die if I try and rev it again.

I can run a full tank of fuel through the thing at partial throttle (if I'm doing light work and I use the throttle trigger stop for starting) without it ever missing a beat. Same thing if I leave it at WOT, never a problem. It only happens if I'm changing throttle position as I trim.

New plug, carb and fuel tank filter are clean as a whistle, carb adjusted well. I'm thinking a fuel problem but it surprises me that it runs so well at WOT if it is leaning out intermittently on me when trying to raise the revs from low speed. I've also tried removing the fuel cap and reinstalling in case it wasn't venting, and no change.

Sometimes I can run an hour of normal trimming and never have the problem. Sometimes 5 minutes. Sometimes it gets so I can't even get it to idle and need to leave it sit for an hour before it's fire right back up but that's only happened once mid season last year.

Curious to see where you guys would start.

Thanks

pen

 

[maple1]

I've got one of those, and this sounds familiar - mine could go wonky like that at any time, then soon come back around. Seems worse at the first of the season, better towards the end. Haven't gotten it out yet this year, but might try some seafoam when I do. I've just been living with it - but if I get around to doing something about it, I think my first plan/effort will be a carb kit. Hoping fresh innards & diaphragms will be the ticket - but yes, these things are troopers. I've given mine basically no maintenance since I've had it, it just gets returned to a shelf up high in my shed when I'm done with it. Sitting in the heat that's up there in the summer can't be all that good for it over time.

 

[ScotO]

I'd check into the crank seals, Pen. A machine that old that's used regularly may be pulling extra air into the crankcase. Does it ever seem to throttle up on its own? Not saying that is the problem for sure, but it is worth looking into.

Also, even though the carb appears clean, you may have something inside one of the small venturis in that carb. Use a micro drill set or even a welders torch tip cleaner to clean the venturis out. Also, replace the fuel lines and fuel filter. You may have a pinhole or small crack in the fuel line causing it to act up. Just a couple of ideas to try out.

 

[pen]

I'd check into the crank seals, Pen. A machine that old that's used regularly may be pulling extra air into the crankcase. Does it ever seem to throttle up on its own? Not saying that is the problem for sure, but it is worth looking into.

Also, even though the carb appears clean, you may have something inside one of the small venturis in that carb. Use a micro drill set or even a welders torch tip cleaner to clean the venturis out. Also, replace the fuel lines and fuel filter. You may have a pinhole or small crack in the fuel line causing it to act up. Just a couple of ideas to try out.

Thanks, I've been nursing things along as the thing does run top notch yet so long as I set the throttle and just leave it there. The trimmer head is about 90% whooped and I've kinda been waiting for that to go as an excuse to tear into the thing then go to the stihl dealer and order some parts.

Machine never throttles up on it's own, really just runs great most of the time.

Think I'm going to start with the fuel lines and filter and see where that goes.

 

[semipro]

I'd check into the crank seals, Pen. A machine that old that's used regularly may be pulling extra air into the crankcase. Does it ever seem to throttle up on its own? Not saying that is the problem for sure, but it is worth looking into.

Also, even though the carb appears clean, you may have something inside one of the small venturis in that carb. Use a micro drill set or even a welders torch tip cleaner to clean the venturis out. Also, replace the fuel lines and fuel filter. You may have a pinhole or small crack in the fuel line causing it to act up. Just a couple of ideas to try out.

Second on this, particularly the carb issues, they're tricky little buggers. As once a professional mechanic I've rebuilt thousands of carbs. I've never had much luck with these guys though. If its a Walbro its probably inexpensive and worth considering replacing it.
I'd also check the fuel tank cap. It should be vented so that air can enter but fuel won't leak out. I'd also check the fuel line where it enters the tank to make sure its not been squeezed down and restricted.

 

[jharkin]

Second on this, particularly the carb issues, they're tricky little buggers. As once a professional mechanic I've rebuilt thousands of carbs. I've never had much luck with these guys though. If its a Walbro its probably inexpensive and worth considering replacing it.
I'd also check the fuel tank cap. It should be vented so that air can enter but fuel won't leak out. I'd also check the fuel line where it enters the tank to make sure its not been squeezed down and restricted.

Third. Dying on throttle up is usually a sign of an overly lean idle mix. Something could be clogged up somewhere. In addition to all those tiny passages and the jets there is typically a small filter screen under the pump diaphragm that can clog.

 

[Eatonpcat]

17 years...That's a good run, Treat yourself to a new trimmer!! LOL

 

[pen]

17 years...That's a good run, Treat yourself to a new trimmer!! LOL

The wife's comment was "don't they make more of those everyday?"

Trimmer works great just a bug to sort out. But to be honest, asking for more years of service out of it is mostly me being nostalgic. I started mowing vacation home lawns around the two lakes I grew up near when I was 12. At 14 I bought that trimmer and it was my first piece of equipment that I bought with my own earned money. I was quite proud at the time and it tends to make me be a bit blind to the fact it isn't new anymore.

pen

 

[Bret Hart]

Not sure if you need it but I have the service manual for the FS40 if you'd like it.

 

[DianeB]

came to DIY looking for a recommendation for a weed wacker, sound like Stihl might be a good one if yours lasted 17 years. We have the standard lawn trim to do, but we are now mowing larger sections of our pasture and need to get out into the pasture to trim under fences so the electric corded models no longer work - need gas powered. At the very end of our pasture we border a train line. That section is now over grown with brush. Would this model help with brush or would we need to get something specfically for that?

 

[maple1]

I hear 'brush' & I picture something like overgrown with alders or small hardwoods. I've never tried one of those blade head things, but a standard string trimmer like the FS won't do much to what I would call 'brush'. Anything much more than the size of say a small young rose bush or raspberry shoot will usually eat up string. Mine gets used for grasses only. I have been tempted to try one of those blade attachments though.

 

[ScotO]

came to DIY looking for a recommendation for a weed wacker, sound like Stihl might be a good one if yours lasted 17 years. We have the standard lawn trim to do, but we are now mowing larger sections of our pasture and need to get out into the pasture to trim under fences so the electric corded models no longer work - need gas powered. At the very end of our pasture we border a train line. That section is now over grown with brush. Would this model help with brush or would we need to get something specfically for that?

Check out the Stihl KM line-up. Tons of different attachments for it (blower, weedwhacker, brushcutter, tiller, powerbroom, edger, hedgetrimmers, etc.) I have a KM55R system, and I love it! I've got a tiller and polesaw for it, and it does the job pretty well. For heavier work they make bigger models, but this one suits my needs fine.

Also, the FS45 and FS55 are great machines too. Pricey, but well-worth the dough IMO.

 

[heat seeker]

I have used the blade on a trimmer, and it worked well for small brush. If you decide to go that way, make sure the machine you buy will take a blade. When I bought mine, the salesman told me it would take a blade. A few months later, when I went to buy the blade, I found out he was wrong.

 

[MasterMech]

Is this what your unit looks like? (Powerhead should be identical)

(broken image removed)

If so, I've got some bittersweet advice for you.

Replace it. You're gonna spend $50+ in parts (head, carb kit, seals, etc.) plus hours of your time to repair a trimmer that even Stihl wants to pretend didn't exist. They have begun running the FS40 model designation again but this time around with the homescaper engine, which is excellent BTW.

Stihl has abandoned the single bearing supported crankshaft design and with good reason, as these units were extremely prone to air leaks via the crank seal (especially as the bearing wears) or rear cover gasket. Not to mention the front mounted recoils were relatively difficult to service and the bleepin' choke mechanism that usually never stays in position.

I will say this about a replacement, the average homeowner looking to just trim his/her yard need not fear for the longevity of Stihl's current HomeScaper trimmers. The golf course I work on has 3 FS55R's and if they can survive four seasons of minimally compensated summer help running them days on end trimming river banks, trees, and bunker edges.... Then they can most certainly take whatever you throw at them. They are the absolute most trimmer $200 will buy by far.

 

[maple1]

That's exactly what mine looks like - although I was mistaken, mine's not a 40 but rather a 35 (maybe? quick look in the semi dark shed). I'm running it until it calves. It only gets used maybe 4 times a year at an hour a pop. It has been a gem compared to what it replaced - I forget now exactly what that was, maybe a Shindaiwa. That thing drove me nuts. The only thing I don't like about my FS is I can't do much of anything for a while after using it because the vibes pretty well put my arms to sleep from the elbows down. Kind of weird - all tingley & shakey with no grip. That might be my arms though - I ain't quite what I used to be.

 

[MasterMech]

mine's not a 40 but rather a 35 (maybe? quick look in the semi dark shed).

FS36, the FS40's far more popular sibling.

 

[StihlHead]

On the Stihl FS85 trimmer that I had, I found that the exhaust muffler screens tended to clog up on me. It was just like you describe, and I thought it was the carb, air filter or seal leaks, but it was just a caked up muffler screen. We used that thing to death and then some, and as far as I know, the ex is still using it.

I found that the Stihl trimmers have high vibration as a group, and I have a Redmax now that I like a lot better.

 

[ScotO]

+2 on the muffler spark arrestor screen. I took it out altogether, and although the trimmer is louder, it runs great.....

 

[StihlHead]

Louder w/o the screen? Seems odd... we have to keep screens on gas power tools in this state for fire requirements. Running saws, blowers and trimmers w/o a spark screen can get you a fat fine.

 

[StihlHead]

BTW: Switch to 100% synthetic premix oil and your saw/trimmer/blower screens will not clog up as much, if at all. We were using dyno when the clogged screen thing happened.

Hey, I crossed over 1000 posts here... geez, I need to get a life!

 

[DianeB]

Check out the Stihl KM line-up. Tons of different attachments for it (blower, weedwhacker, brushcutter, tiller, powerbroom, edger, hedgetrimmers, etc.) I have a KM55R system, and I love it! I've got a tiller and polesaw for it, and it does the job pretty well. For heavier work they make bigger models, but this one suits my needs fine.

Also, the FS45 and FS55 are great machines too. Pricey, but well-worth the dough IMO.

checked it out online and may go to see it at the local dealers. Could see they have home-use and commercial models. The KM55 R home use and you seem very hnappy with it. Have you ever tried the tiller attachment? That was interesting. A neighbor rototills our garden in the spring and we spend the summer hoe'ing it. The little tiller might be the trick to keep up with it.

 

[DianeB]

I have used the blade on a trimmer, and it worked well for small brush. If you decide to go that way, make sure the machine you buy will take a blade. When I bought mine, the salesman told me it would take a blade. A few months later, when I went to buy the blade, I found out he was wrong.

I did see that some models can change out the string for a blade - got to watch those sales people.

 

[pen]

On the Stihl FS85 trimmer that I had, I found that the exhaust muffler screens tended to clog up on me. It was just like you describe, and I thought it was the carb, air filter or seal leaks, but it was just a caked up muffler screen. We used that thing to death and then some, and as far as I know, the ex is still using it.

I found that the Stihl trimmers have high vibration as a group, and I have a Redmax now that I like a lot better.

I pulled that exhaust screen off on the advice of a family member the first year I owned the trimmer.

When I finally get sick of nursing this guy along I think I'm going to take MasterMech's advice. Not sure if I'll go for another stihl or not but I haven't found another trimmer that I've run I like as much as a stihl yet. Really good friends with the husqvarna dealer but I've tried two husky trimmers that would be the equivalent or larger than my stihl and didn't like them nearly as much. They had plenty of power wound right up, but didn't have the torque to trim at lower rpm, plus they were difficult to even operate at a steady 1/2 throttle for delicate spots like I can with this stihl.

I'd like to run a new one and see how it compares. The one stihl dealer is also a rental shop so I'm sure I can take a few for a spin if I wanted.

I can't say enough for this old girl. Still starts the same as it did brand new. 3 pumps primer, choke full, pops second light pull. Take choke off, sputters next pull, runs steady 2nd light pull. Every start, every time for 17 years.

When I buy a piece of equipment I like it to last. Sure can't complain here and would probably be foolish not to get another stihl.

pen

 

[semipro]

I bought a 4 stroke trimmer for our 5 acres and will never go back to 2-stroke.
A little heavier but way quieter, torquey, hardly any stink, and no oil mixing.
It uses the same engine as Honda's smaller generators.

 

[ScotO]

checked it out online and may go to see it at the local dealers. Could see they have home-use and commercial models. The KM55 R home use and you seem very hnappy with it. Have you ever tried the tiller attachment? That was interesting. A neighbor rototills our garden in the spring and we spend the summer hoe'ing it. The little tiller might be the trick to keep up with it.

I use that little tiller on my KM55R all the time, it works fantastic for getting around your flower and garden beds. It's not the best thing in the world for breaking up hard, compacted soil....but works great in flower beds and gardens.....

It WILL break fresh dirt, but you'd be better off spading it or using a big tiller for the hard stuff.